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Home > About USPS & News > Forms & Publications > Postal Periodicals and Publications > Publications > Publication 8A - Address Change Service - Traditional > Participation Requirements > USPS Standards for Check Digit Computation
ACS participants using the keyline option must use one of the following
algorithms to compute the check digit:
MOD 10
1. Consider the keyline as an ASCII string of 3 to 15 characters. Remove
any spaces before beginning calculation.
2. Convert the remaining characters to pure numeric values by zeroing
out all but the lower four bits. This yields the following values for each
alpha character (numeric characters retain their value):
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A = 1
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E = 5
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I = 9
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M = 13
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Q = 1
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U = 5
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Y = 9
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B = 2
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F = 6
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J = 10
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N = 14
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R = 2
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V = 6
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Z = 10
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C = 3
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G = 7
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K = 11
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O = 15
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S = 3
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W = 7
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blank |
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D = 4
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H = 8
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L = 12
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P = 0
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T = 4
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X = 8
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/ = 15
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3. Multiply the value at each odd-numbered position (first position, third
position, fifth position, etc.) by a weighting factor of 2.
4. Add the digits (not the actual value) in all positions. For example, if the
second position of the keyline is "L," whose value is 12, add the digits 1
+ 2 (not the value 12) to the sum.
Note: The value of the letter "N" in an odd position, when
weighted, becomes 14 x 2 = 28. The digits 2 and 8 are added,
yielding 10. This sum is then further rendered as 1 + 0, not the
integer 10.
5. Subtract the right-most digit of the sum from 10, yielding the check digit.
Note: If the right-most digit of the sum is 0, the check digit will be 0
Example 1. Computation of Check Digit for Keyline JLSTMS6796
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Character
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J
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L
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S
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T
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M
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S
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6
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7
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9
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6
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Position
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1(O)
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2(E)
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3(O)
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4(E)
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5(O)
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6(E)
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7(O)
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8(E)
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9(O)
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10(E)
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Value
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10
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12
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3
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4
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13
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3
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6
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7
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9
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6
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Weighted Value
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20
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12
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6
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4
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26
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3
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12
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7
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18
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6
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Sum
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2+0
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+1+2
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+6
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+4
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+2+6
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+3
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+1+2
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+7
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+1+8
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+6 = 51
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Right-most Digit of Sum: 1
Check Digit: 10-1=9
Complete Keyline: JLSTMS67969
Example 2. Computation of Check Digit for Keyline TMS1112/62
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Character
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T
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M
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S
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1
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1
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1
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2
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/
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6
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2
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Position
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1(O)
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2(E)
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3(O)
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4(E)
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5(O)
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6(E)
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7(O)
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8(E)
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9(O)
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10(E)
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Value
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4
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13
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3
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1
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1
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1
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2
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15
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6
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2
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Weighted Value
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8
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13
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6
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1
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2
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1
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4
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15
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12
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2
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Sum
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8
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+1+3
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+6
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+1
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+2
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+1
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+4
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+1+5
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+1+2
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+2 = 37
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Right-most Digit of Sum: 7
Check Digit: 10-7=3
Complete Keyline: TMS1112/623
Example 3. Computation of Check Digit for Keyline 218XN91LMS
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Character
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2
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1
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8
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X
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N
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9
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1
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L
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M
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S
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Position
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1(O)
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2(E)
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3(O)
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4(E)
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5(O)
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6(E)
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7(O)
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8(E)
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9(O)
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10(E)
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Value
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2
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1
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8
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8
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14
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9
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1
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12
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13
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3
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Weighted Value
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4
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1
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16
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8
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28
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9
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2
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12
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26
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3
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Sum
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4
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+1
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+1+6
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+8
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+1+0
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+9
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+2
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+1+2
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+2+6
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+3 = 46
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Right-most Digit of Sum: 6
Check Digit: 10-6 = 4
Complete Keyline: 218XN91LMS4
The following are additional examples of valid keylines that you may use for
testing your algorithm. In each case, the check digit is the last character.
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ABC999NN//32
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OTUBIKALAM8
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ANIT0281
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SEIRAMT4
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0012////8
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14024981490
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811NIS0001119
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AEIOUW0
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12345678907
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DCS14020
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09876543213
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blank |
Note: Although optional for COA information, keylines are required if the
mailer wants to receive electronic ACS nixie notifications. Also, each
keyline must end with a check digit correctly calculated using the Postal
Service standard for check digit computation.
MOD 10 REVERSE
Computation of the MOD 10 Reverse is the same as the standard MOD 10
with one exception. MOD 10 Reverse is computed starting right to left instead
of left to right.
MOD 21
ACS participants using the keyline option must use the following algorithm to
compute the MOD 21 check digit:
1. Consider the keyline to be an ASCII string of 3 to 15 characters.
Remove any spaces before beginning calculation.
2. Convert the remaining characters to pure numeric values using the
conversion table below.
3. The positional value of a character is determined by multiplying the
numerical value derived from the conversion table by one more than
the length of the keyline without the check digit for the first character,
the length of the keyline without the check digit for the second
character, one less than the length of the keyline for the third character,
and so on. Thus, for a keyline without a check digit of the minimum
length of three characters, the numerical value of the first character
would be multiplied by four and the numerical value of the second
character would be multiplied by three. For a keyline without a check
digit of the maximum length of 15 characters, the numerical value of the
first character would be multiplied by 16, the numerical value of the
second character multiplied by 15, the numerical value of the third
character multiplied by 14, and so on up to multiplying the value of the
15th character by 2.
4. The positional values are added up. This total sum is divided by 21 and
the remainder obtained. Multiply this number in whole number form (do
not round up) by 21. Subtract the resulting product from the total sum.
The difference is then subtracted from 21 yielding a numeric value,
which should then be converted to the letter using the conversion table
shown.
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/ = 0
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F = 6
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L = 12
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R = 18
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X = 24
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0 = 27
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A = 1
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G = 7
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M = 13
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S = 19
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Y = 25
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1 = 28
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B = 2
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H = 8
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N = 14
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T = 20
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Z = 26
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2 = 29
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C = 3
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I = 9
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O = 15
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U = 21
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blank |
3 = 30
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D = 4
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J = 10
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P = 16
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V = 22
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blank |
4 = 31
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E = 5
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K = 11
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Q = 17
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W = 23
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blank |
5 = 32
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| blank |
blank |
blank |
blank |
blank |
6 = 33
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| blank |
blank |
blank |
blank |
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7 = 34
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| blank |
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blank |
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8 = 35
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| blank |
blank |
blank |
blank |
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9 = 36
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Example 1. Computation of Check Digit for Alphanumeric Keyline 025G
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Character
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0
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2
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5
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G
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Numeric Value
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27
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29
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32
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7
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Multiply by
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5
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4
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3
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2
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Positional Value
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135
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116
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96
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14
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Total Sum of Positional Values
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135+116+96+14
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361
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Total Sum Divided by 21
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361/21
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17.19
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21 Multiplied by the Whole Number
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21 x 17
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357
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Subtract Product From Total Sum
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361 - 357
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4
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21 - Remainder
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21 - 4
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17
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Check Digit
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17 = Q
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Q
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| blank |
blank |
Complete Keyline:
025GQ
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Example 2. Computation of Check Digit for Alpha Keyline QWERTY
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Character
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Q
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W
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E
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R
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T
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Y
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Numeric Value
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17
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23
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5
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18
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20
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25
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Multiply by
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7
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6
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5
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4
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3
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2
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Positional Value
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119
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138
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25
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72
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60
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50
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Total Sum of Positional Values
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119+138+25+72+60+50
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464
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Total Sum Divided by 21
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464/21
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22.10
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21 Multiplied by the Whole Number
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21 x 22
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462
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Subtract Product From Total Sum
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464-462
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2
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21 - Remainder
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21 - 2
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19
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Check Digit
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19 = S
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S
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| blank |
blank |
Complete
Keyline:
QWERTYS
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Example 3. Computation of Check Digit for Numeric Keyline 123456
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Character
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1
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2
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3
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4
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5
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6
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Numeric Value
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28
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29
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30
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31
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32
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33
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Multiply by
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7
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6
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5
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4
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3
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2
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Positional Value
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196
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174
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150
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124
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96
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66
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Total Sum of Positional Values
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196+174+150+124+96+66
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806
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Total Sum Divided by 21
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806/21
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38.38
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21 Multiplied by the Whole Number
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21 x 38
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798
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Subtract Product From Total Sum
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806 - 798
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8
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21 - Remainder
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21 - 8
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13
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Check Digit
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13 = M
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M
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| blank |
blank |
Complete
Keyline:
123456M
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