A correctly folded protein
adopts a very specific tertiary structure. This is the level at which secondary structure
units such as alpha helices
and beta sheets
assemble into a specific three dimensional arrangement. This is also known as the fold.
Structural biologists have come up with a variety of colorful names for different sorts of tertiary folds such as beta sandwiches, helical bundles, beta barrels, greek-key motifs, propellors and many others.