HOW TO MEASURE AN ENVELOPE
It is traditional to state the measurements of an envelope in terms of width X length (excluding the flaps).
OPEN END, OPEN SIDE
In envelope terms, the width is also known as the end. The length is called the side. We refer to their products either “open end” or “open side”. These categories refer to where the opening (flap, throat) of the envelope is.
LIST OF ENVELOPE SIZES and TYPES