#10 envelope: Standard size envelope often used for business correspondence. Sometimes called a commercial or official style envelope.
Announcement envelopes: a style of specialty envelope with a straight flap used for invitations and personal stationery. Also known as A-style.
Aspect ratio: the length of an envelope divided by the width. Used by the USPS to determine whether an envelope shape is rectangular. Rectangular envelopes are eligible for the best postage rates.
Baronial envelope: a style of specialty envelope with a pointed flap used for invitations and personal stationery.
Blanks: a flat sheet of paper cut to the proper size and shape to be converted into an envelope.
Booklet envelope: envelope with the flap on the long side.
Catalog envelope: envelope with the flap on the short side.
Commercial envelope: a category of envelopes used for business correspondence, direct mail, and invoicing/payment.
Converting: making an envelope from an envelope blank cut from a flat sheet of paper.
Franking: affixing postage to an envelope.
Lettersheet: illustrated sheets of paper that could be sealed and sent through the mail with no envelope.
Mulready: illustrated postal lettersheets and envelopes used briefly in Britain in 1840. Named for the artist, William Mulready.
Peel and seal: a type of envelope closure method.
Penny Black: the first British adhesive postage stamp. Printed using the Jacob Perkins printing process to deter counterfeiting and forgery.
Postal wrapper: a precursor to the envelope, introduced in 1840 in Britain. Sold as a sheet of 12 printed in a diamond pattern, requiring the user to cut them out, fold, and affix a seal.
Press and seal: a type of envelope closure method.