The carrick bend is a knot used for joining two lines. It is useful when you need a bend formed from heavy rope. It will not jam even after carrying a significant load.
The carrick bend is generally tied flats as shown in the first picture below. The as it is pulled tight it collapses into a different shape. This is known as capsizing. This capsized form is both secure and stable once tightened but it is bulkier than the seized. The seized form is used often because it is more pleasing to the eye and is often used for decorative purposes.
If the knot will be carrying an important load you should tighten the knot, PRE_LOAD it prior to use to avoid the knot slipping when loaded up for the first time.