What is a Side Seam envelope and how does it differ from a Diagonal Seam envelope?

Most invitation "A"-sized envelopes have side seams. Think A2, A7, etc. Most commercial sized envelopes have diagonal seams. Think #9 regular, #10 regular, etc. What is the difference?. Side Seam: The back of the envelope is put together with two vertical glue seams (as opposed to three triangular flaps) which give a more finished look to the envelope. Diagonal Seam: Fitting to their name, seams run diagonally across the back. The back of the envelope is put together as three triangular flaps.