Commercial and official:
These envelopes normally have a commercial flap (standard flap), however some manufacturers offer a square flap version as well for certain sizes.
#6 1/4 envelope – 3.5625 x 6.25
#6 3/4 envelope – 3.625 x 6.5
#8 5/8  envelope – 3.625 x 8.625
#7 3/4  envelope – 3.875 x 7.5
Monarch – 3.875 x 7.5
#9  envelope – 3.875 x 8.875
#10 envelope – 4.125 x 9.5
#11 envelope – 4.5 x 10.375
#12 envelope – 4.75 x 11
#14 envelope – 5 x 11.5

Announcement envelopes:
These envelopes have a square flap.
A-2 envelope - 4.375 x 5.75
A-6 envelope – 4.75 x 6.5
A-7 envelope – 5.25 x 7.25
A-8 envelope – 5.5 x 8.125
A-10 envelope – 6 x 9.5

Baronial Envelopes (abbreviated “Bar”):
These envelopes have a baronial (pointed) flap.
#4 Baronial envelope – 3.625 x 5.125
#5 Baronial envelope – 4.125 x 5.5
#5 1/2 Baronial – 4.375 x 5.75
#6 Baronial – 4.75 x 6.5
Lee – 5.25 x 7.25

Remittance envelopes:
These envelopes have a statement or return flap
6 1/4 Remittance envelope – 3.5 x 6
6 1/2 Remittance envelope - 3.5 x 6.25
6 3/4 Remittance envelope – 3.625 x 6.5
#9 Remittance envelope – 3.875 x 6.5

Other Envelope Facts:

• A Booklet envelope has the flap on the long side. For example, on a 9×12 booklet, the flap is on the 12 inch side.
• A Catalog envelope has the flap on the short side. For example, on a 9×12 catalog, the flap is on the 9 inch side.
• A converted envelope is when the envelope is printed on a flat sheet and then made into an envelope. This is opposed to printing on a preconverted envelope. A converted envelope is most economically feasible at larger quantities.
• A square flap envelope with rounded corners is called a pictorial flap envelope. In most cases this is only available with a converted envelope.