Notes and letters fit in envelopes better and look more professional when the paper is folded into thirds. The technique varies slightly for business envelopes and smaller envelopes, but the 1/3 folding rule still applies with slight variation.
A standard 8-1/2-inch-by-11-inch piece of paper fits perfectly in a business envelope when folded into thirds. Set the paper on a flat work surface with the writing facing upward and the length running side to side. Fold the left end over toward the right, lining up the end 2/3s the way down the paper. Fold the right end over to the left, lining up the end with the crease from the first fold. Slip the note into the envelope with the folded sides facing the envelope front.
The address at the top of a letter is positioned so it can be seen through the plastic window in some envelopes. When using an envelope with a window, fold the top of the letter, which is the right side, backward so the address faces out. Slip it into the envelope with the address peeking through the window.
Small envelopes can be used for professional notes, but look best when the letter is folded in thirds. Fold the bottom edge of the paper up toward the top, stopping about 1 inch shy of the top edge. Fold the left and right sides over into vertical thirds, similar to folding for business envelopes. Insert the paper in the small envelope with the folds facing the front of the envelope.