Alpha-helix and beta-sheet conformations are the two main types of secondary structure of a protein molecule. According to the primary protein structure its secondary structure can be of one type or the other.

In the alpha-helix structure the polypeptide curls longitudinally by the action of hydrogen bonds forming a spiral, or helix. In the beta-sheet confirmation the protein is more distended and the hydrogen bonds form a zig-zag-shaped protein structure called B-strand. Many assembled beta strands make a beta-sheet.