Crease [noun]: the indentation caused by a fold
In our various discussions of various delightful pieces of origami, we here at Ourigami Origami Informational Content Recipes Inc. LLC Eastern Massachusetts division often tell you to do things like “fold so that the creases lie on top of each other” and “fold from the edge to the crease”. But what do we really mean? Well, as you might have heard, a crease is the line that’s the result of a fold. So, if you “fold so that the creases lie on top of each other” you are creating a new fold halfway between the two previous folds, and “folding from the edge to the crease” is creating a fold halfway between the previous fold and edge. However, this usually comes with a couple assumptions - specifically, that the fold is perfectly sharp and linear - and an important distinction when folding.
Depending on the force you apply when folding, the crease will turn out very differently. If you apply a large amount of force, your crease will be essentially flat. However, if you don’t, it’ll be a glorified bend in the paper, with the radius of the bend depending on exactly how much force you put. Sometimes, this is desirable, as when you want to bend over another, sharper fold.
Another important distinction is mountain vs. valley folds. These terms are really relative, they merely depend on the way you hold the paper. A “valley fold” means the fold is pointing away from you and a “mountain fold” means the fold is pointing towards you. When there are two of the same type being creased to each other, you usually have slightly less precision than when there are two of different types creased to each other as then you can fit one crease into the other.
