Beta turns (ß turns, ß-turns, ß-bends, tight turns, reverse turns) are very common motifs in proteins and polypeptides. Each consists of four amino acid residues (labelled i, i+1, i+2 and i+3). They can be defined in two ways: 1. By the possession of an inter-main-chain hydrogen bond between the CO of residue i and the NH of residue i+3. Alternatively, 2. By having a distance of less than 7Å between the Ca atoms of residues i and i+3. The hydrogen bond criterion is the one most appropriate for everyday use, partly because it gives rise to four distinct categories; the distance criterion gives rise to the same four categories but yields additional turn types. Two websites are available for finding and examining hydrogen-bonded beta turns in proteins, Motivated Proteins: [1]    and PDBeMotif [2] Beta turns defined by hydrogen bond criterion:

The hydrogen bond criterion for beta turns, applied to polypeptides whose amino acids are linked by trans peptide bonds, gives rise to just four categories, as shown by Venkatachalam in 1968. They are called types I, II, I’ and II’. All occur regularly in proteins and polypeptides but type I is commonest, because it most resembles an alpha helix, occurring within 3/10 helices and at the ends of some classic alpha helices.

A turn is an element of secondary structure in proteins where the polypeptide chain reverses its overall direction. For beta turns go to Beta turn.