Origami diagrams are visual representations of the instructions that help you build various Origami sculptures. When you look at those fantastic paper sculptures that people have made, it is quite hard to imagine that someone may have made that out of, in most cases, a single sheet of flat paper. However, to enable you to understand the steps behind making something out of paper, standardized Origami diagrams are used to hold your hand as you go along.
In the early days, when books about Origami designs came out, the steps were generally shown through before-after images. While these images showed how the paper should look after you made the change, it made it extremely difficult to understand exactly how that change needed to be made. This process continued until Akira Yoshizawa came up with a symbol to indicate what needed to be done and how.
These symbols didn't just tell you what the turns were and how the paper should look after they were made; they were explaining the process of creating paper sculptures, identifying the common patterns that would come through them, creating standardized bases that would help you take things ahead and much more. Yoshizawa thought of almost every single thing that could be done to manipulate paper without cutting or pasting anything. Whatever he missed out on, Samuel Randlett put in.
As a result, the Yoshizawa-Randlett system is considered the gold-standard in the world of Origami diagrams.
The Origami Lines
There are six basic kinds of lines that are denoted through different symbols in Origami diagrams. The thick bold lines are the kinds that are used to show the actual outlines of the paper. They are used to denote the edges that would be visible when viewing from the angle shown in the diagram. A dashed line is a symbol of a valley fold while a dash and dot line is meant to indicate a mountain fold.
In some cases, a mountain fold is also indicated by a dash and two-dot line.
A thin line, in Origami diagrams, is a sign to show where a crease has been made in the paper while a dotted line is a symbol of a fold/crease that has been made on the other side of paper, i.e. the side that is away from your view. These symbols are the only things you need to know about Origami diagram lines and using just these 6 line-symbols, you can decode thousands of diagrams and understand what to do, although they aren't the only elements you need to know.
The Origami Arrows
Arrows are another popular symbol used in Origami diagrams and are used to indicate certain types of folds. In most cases, these folds are the simplest versions wherein you don't need to make special adjustments or combine two or more things to make the fold. The simplest amongst these is a simple arrow made at the end of a solid line - this represents a valley fold. The valley fold arrow has a line-drawn arrow head and not one shaped like a triangle.
If the arrow-head were shaped like a triangle, then it would become an indicator of a pull out or an unfold symbol. However, if you only had half an arrow head, you would find yourself looking at a mountain fold symbol. The half-arrow head, in a mountain fold symbol, is usually on the upper side of the solid line.
Fold-unfold arrows are the kinds come with a line-drawing arrow head on one side and a triangle arrow head on the other side of a solid line. This is used to indicate the direction in which the paper has to be, first, folded and, then, unfolded. Finally, if you found a number in a circle, in the middle of the stem of the arrow, then that would indicate the number of times you would need to repeat the fold. This number, in the middle of a circle, would appear as if the arrow were passing through it and would be the same for any kind of fold, valley, mountain or unfold!
Other Origami Symbols
While these arrows and symbols may tell you almost everything you needed to know, there are other actions that too require notations on Origami diagrams. One of the most common ones, for these, is the turn-over arrow, which is a regular valley fold arrow but with a loop on the step. This indicates that you need to turn your paper around. Then there's the rotate symbol that has two semi-circular arrows with the angle of rotation mentioned in the middle of it.
"Push the paper" is indicated by a thick arrow shape without any colour filled in while a thick solid arrow, with a colour filled in, would indicate the place where you needed to put your finger. There are plenty of such symbols, such as for the number of times you need to repeat steps, the viewing angle, where and how to arrange layers, and much more. Understanding these symbols will help you understand Origami diagrams better and even if there are instructions alongside, you will find these Origami diagrams to be more helpful than anything else.