I mail our calendars each year to my regular "supporters" who work with me on physics textbooks and on sending letters to despots around the world. They are twelve-by-twelve inch calendars and as such rather expensive to mail. But I have learned to fit each into one of these envelopes folded upon sealing to be 12-by 11 3/4 inches which saves a lot of postage. Also Staples sells both number 10 envelopes and number 9 envelopes both good for business letters and allowing preaddressed mail-back or mail-on envelopes to fit conveniently inside letters I send to people. My life is easier with a reasonable supply of both sizes on hand at all times.