Table A2-21 First-Class Mail -- Total Envelopes by Sector (Percentage of Pieces) Postal Fiscal Years 1987, 2006 and 2007 (Diary Data)
Sector	1987	2006	2007
Household-to-household	9.4	6.4	4.6
Household-to-nonhousehold	12.3	13.9	15.1
Nonhousehold-to-household	39.2	46.9	41.2
Unidentified incoming	0.6	2.9	2.2
Unidentified outgoing	1.4	0.8	1.0
Subtotal	62.9	71.0	64.1
Nonhousehold-to-nonhousehold	37.1	29.0	35.9
Total	100.0	100.0	100.0
Note: Totals may not sum due to rounding.
Base: RPW Total, First-Class Envelopes.