OFFSET PAPER - Also known as book paper. General description of any paper primarily suited for offset printing. Can be coated or uncoated. Characterized by strength, dimensional stability, lack of curl and freedom from foreign surface material. Finish can be vellum or smooth.

OFFSET PRINTING - Also know as web offset or lithography. Offers highest degree of precision, clarity, and quality. Uses screens of 133 lines or more, printed flat sheet, then folded. Can be solid PMS, in multiple colors and process printing.

OPACITY - Paper property that measures the degree to which paper stops light from passing through. The more opacity a paper has, the less show-through it permits of the envelope contents. Inside tints can be used to compensate for low-opacity papers.

OPEN END - An envelope term describing an envelope with the seal flap on the shorter of the two dimensions.

OPEN PANEL - A term used when any envelope with a window does not have any type of clear window patch material affixed over the window opening from inside the envelope.

OPEN SIDE - An envelope term describing an envelope with the seal flap on the longer of the two dimensions.