| A class of secondary structure that is flat and stretched. The protein backbone is in a so-called 'extended' conformation. The result is a kinked sheet like corrugated cardboard. Residues project either side of the sheet, alternately up and down along the chain.
Antiparallel sheet is formed from sequential strands:
_ C
| | |
/ \ | / \
| | |
| | |
| \ / |
N |_|
While parallel sheet requires crossover connections, a beta alpha beta motif. Since these can be left or right handed, it would seem reasonable to assume an equal distribution of each. However, relatively few parallel sheets are left handed, a notable example is subtilisin. | 
