Does a #9 envelope fit inside a #10 envelope?

Yes. A #9 fits inside a standard #10 (4 1/8" x 9 1/2") without folding, which is why it's the most common return envelope.

Shop Custom #9 Envelopes (3 7/8″ x 8 7/8″)

Other questions in #9 Envelope Printing FAQs