Picking up a pattern piece can be like trying to read a foreign language. There are many basic rules of thumb on pattern symbols. They may vary slightly from company to company, but most are universal. Always refer to the manufacturer's pattern directions.
On almost every commercial pattern, there is a key to the symbols that are on your pattern pieces. This should be found either on pattern tissue or on the direction sheet. Refer to it when ever you are in doubt.This symbol key will vary by company and may change over time, to prevent costly errors, do a quick refresher every time you open a pattern.
Pattern Adjustment Lines
These are lines included on the pattern piece for lengthening and shortening the pattern piece. Always refer to the pattern directions to alter in these locations.
These are lines included on the pattern piece to make sure you cut the fabric out on the proper grainline for the way the garment is intended to hang. When measuring from the salvage edge measure more then one area of the line. A 1/4" difference from one end of the line to the other, magnifies it self in the finished garment. If you are teaching someone to sew, it is a good idea to extend this line and allow them to measure the difference from the salvage edge.
Place On Fold Line
This line indicates that the edge of the pattern piece must be placed on a fold of fabric. The edge of the pattern piece is usually indicated as a broken line, to remind you not to cut on that edge. If you cut this edge, there is no seam allowance and trying to join these will change the way the pattern fits, as well as how other pieces will join to this piece.
These are usually a solid black line on the outer edge of the pattern piece.
Some pattern companies do not include a seam allowance on the edge of your pattern pieces. Be sure to check the directions to make sure it is included.
On multi sized patterns there will be more then one cutting line in many areas of the piece. If you are going to use the pattern again for a different size, it is wise to trace the pattern piece and save the original.
Notches are used to line up two or more pieces of fabric that you will be joining together. Notches are symbolized in various sizes, from single to quadruple. Larger notches always refer to the back side of the garment, which help keep fronts and backs straight in your mind and prevent mistakes.Notches can be cut into the seam allowance, however when working with beginners it is advisable to have them cut the notch outside of the seam allowance as demonstrated by the red lines in the diagram.
Not all pattern pieces include stitching lines. When they are included they are a broken line, indicating the areas that will be stitched together.These are a guide so you can see where a line of stitching intersects and where you will not be stitching. It is not recommended that you mark every stitching line on to your garment. This would result in excess handling of your fabric.
Dots are made in various sizes by the pattern companies. These must be marked onto your fabric. They indicate starting and stopping points for stitching, as well as points to match up markings for things like darts. When you are working with multi sized patterns, be sure you transfer the marking for the size you are working with.
More Information on Understanding Patterns