A schematic representation of secondary structure of a protein in which α-helix is rendered as a circle, β-sheet as a rectangle and connecting sequences as lines. The circles and rectangles are arranged in two dimensions to approximate their true orientation in space. Proteins fall into four classes: all-α, which have only helix secondary structure; all-β, which have only sheet; α+β, which have regions of both helix and sheet; and α/β, which have approximately alternating helix and sheet.