Envelopes are described with the shortest dimension first. But this doesn't distinguish height from width. Therefore, if an envelope is placed before you with its seal flap at the top, the dimension from the bottom fold to the top fold is the height, and the dimension from the left to the right is the width. Considering the flap position is the first step in describing envelope dimensions.

Open Side (O.S.)
Open side envelopes have the opening or seal flap on the long dimension. There are various names for these: commercial, official, drug, pay, booklet, baronial, announcement, etc.

Open End (O.E.)
Open end envelopes have the opening or seal flap on the short dimension. Catalogs and coins are the principal open ends in stock at Western States.