In parallel beta-sheets the strands all run in one direction, whereas in antiparallel sheets they all run in opposite directions. In mixed sheets some strands are parallel and others are antiparallel. It emphasises the highly regular pattern of hydrogen bonds between the main chain NH and CO groups of the constituent strands. Beta-sheets are very common in globular proteins and most contain less than six strands. No preference for parallel or antiparallel beta-sheets is observed, but parallel sheets with less than four strands are rare, perhaps reflecting their lower stability.
Sheets tend to be either all parallel or all antiparallel, but mixed sheets do occur.
This twist is left-handed as shown below. The overall twisting of the sheet results from a relative rotation of each residue in the strands by 30 degrees per amino acid in a right-handed sense. Parallel sheets are less twisted than antiparallel and are always buried.