Elastic

You can use elastic to make gathers. Elastic is not limited to the edge of a pattern, it can be used anywhere. Elastic in the real world looks like a fabric tape, it comes in all different widths and it has rubber or some other stretchy thread woven in it. No matter how far you pull elastic, it will come back to its original length.

When you use elastic, you can choose how full the gathers are by deciding on the ratio of elastic to fabric.

For example, you may have a piece of fabric that is 10 cm wide and you want it to be 5 cm wide after it is gathered. So you cut a piece of elastic that is 5 cm long un-stretched. You stretch the elastic out to 10 cm, pin the fabric to it and sew. When you release it, the fabric is now 5 cm long. 

In the image above, the large pattern is sewn to another one at the top. The width is 100cm. Elastic has been applied to the bottom of the pattern with a length of 50cm or 50% gathered. Another reference pattern is shown at the bottom and it is 50cm wide. You can see that the edge with elastic is now 50cm wide, just like the reference pattern.

If you have a garment that needs to be pulled in at the waist so it doesn't fall down, like a skirt or pants, elastic is a great choice. It can also be used to reshape sleeves and collars.

Shirring

Shirring is gathering fabric either by pulling threads or using elastic. What makes shirring unique is that it is always done in multiple parallel rows.

Gathering

Gathering is drawing fabric together by pulling on a thread of stitching. You gather one edge until it matches another in length and then sew them together.