Mountain crease: a fold which creates a peak; a fold that faces you.

Valley crease: a fold to the front, which creates a valley or hollow place; a fold that faces away from you.

Crease: a sharpening of the fold, done by pressing a a thumbnail or ruler along the edge of a previously measured fold.

2torial #0855: Learn2 Make Basic Origami (Continued) Project 1: Create a bird Make the wings   Begin by clearing a place to work. If you've washed the working surface, make sure it's thoroughly dry. Starting off with soggy origami paper would be inauspicious. Place your paper so that the prettiest side faces down. (This will give your bird a plain body, but fancy head, neck and wings.) You'll be doing mountain fold and valley folds in origami--see Keywords for more info. Mountain fold your square along a diagonal and then unfold it, leaving it flat. The plain side should still face up. Now orient the square as a diamond, with the crease line running from top to bottom. Use a pencil to label very lightly the left corner as A. Label the right corner as B.   Fold corners A and B, so they meet in the center of your diamond. Crease these folds with a fingernail or a ruler, checking to make sure the new corner you form at the top remains neat. You can label that corner C, if you like. Project 1 - Step 2