If there were no written instructions, would you be able to put a sewing pattern together? This question isn't meant as a quiz for the highly intelligent or for only the practiced seamstress. If you know what all the lines and markings on your pattern are for, your answer should be a resounding YES.These lines and symbols are a pattern makers way of telling you how to assemble the pattern with as few words as possible. You've seen them many times; they lie upon the edges of the cutting line, seam lines, and inside the pattern itself.
Notches are marks found on the cutting line. What they look like will depend upon the program that is making the pattern. They can be triangles, filled or not, or look like clip marks. We use notches ONLY when the shape of a pattern pieces look like another, or if a piece could be easily turned around, or on long seams where stretching or easing could use some guidance.
Symbols come in various types (circles, squares, triangles, lines) and are for interior construction, such as pleats, gathers, ease, and so forth. A symbol usually has a matching partner, and that's why there are so many types. This way we can put circles to circles and squares to squares. Everything should match up perfectly, and sometimes we will add words to the pattern to tell you what that line is for!
I'll use the hip sash section of the Myrtlewood pattern to show you what I mean. Here are two pattern pieces that go together, Piece A and Piece B. One is laid on top of the other. You can't tell that right away, as they are both very different shapes. Pattern Piece A actually has a back piece to it, but we are working with the front half for this example.
On this piece A, you should see darts, notches, symbols, a slash line, and a bent arrow grain line which means you need to cut it out on the fold line, with the grain. 
On piece B, you should see pleat fold and match lines, as well as the direction to fold the pleat, symbols, notches, and a diagonal arrow which means that this pattern piece should be cut on the true bias. Bias has the most stretch in the fabric, and if it isn't cut out with the right grain, the pattern piece will not stretch or lay properly. Because these pieces are sewn together on top of each other, the trick is to identify what goes to where. It would be easy to get them mixed up, or one upside down perhaps. It's like putting together a puzzle, and a good idea to figure out how it goes together before you start sewing. If something doesn't match up, it's your cue that you may not have it right.
Here is a view of the front of that pattern a bit larger.
Can you tell what goes to what?
In the next diagram, I've identified what goes to where. Center Front and Center Back notes let you know where the pattern matches in front and back.
Clearly paying close attention to your pattern pieces is essential but you also want the item to fit exactly. If you are working with a pattern that doesn't quite match your proportions, explore my Pattern Resizing Tutorials, Part 1: Small to Large or Part 2: Large to Small.
I challenge you to sew your next pattern together without written instructions. Did you get the messages that the pattern maker was sending you?
