A letter in an expression stands for a number: an unknown number hiding in an envelope, or any number from a set. "Subtract y from 5" is 5 - y, "3 more than twice n" is 2n + 3. Expressions have parts with names: terms, factors, coefficients, sums and products. To evaluate an expression, put the number in for the letter and follow the order of operations, as in the perimeter 2l + 2w or the volume s³. The crew builds envelope puzzles and card frames at the Puzzle Post.