Although envelopes are measured from the shortest dimension first, this practice does not distinguish the height from the width. Therefore, if the 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. The dimension from the left to right is the width. Simply considering the position of the flap is the first step in describing envelopes.



Measuring a V-Shaped Bottom Expansion Envelope: