The size of a typical #10 envelop is 9.5 x 4.125 inches
…and if you purchase a windowed size, the window placement will be a half-inch from the bottom and 7/8 inches from the left side.
The #10 envelope is used for business purposes. It can fit a letter when it is folded into three panes (tri-folded).
This envelope is used by a lot of businesses to send letters and invoices.