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How to put a stamp on an envelope
Check the size of your envelope.
This should be marked on the packet of envelopes, or on the envelope itself. Size 14 envelopes measure 5 " x 11. 5 " and are considered the standard size.
They will be rectangular in shape and will often be sold in packages at your local post office. You can also mail a letter in an envelope that is smaller than a size 14, such as a size 10 envelope (4.125 " x 9.5 "), with a regular stamp.