Figure 6.3 and Figure 6.4 illustrate actual and theoretical repeating secondary structures of polypeptides. Figure 6.5 illustrates how helices having an integral number (n) of amino acid residues per turn of the helix would look. The convention is to use positive values of 'n' for right-handed helices and negative values of 'n' for left-handed helices. Thus, the structure in Figure 6.5 with n = -3 is a left-handed helix with 3 amino acids per turn of the helix. Notice that values of 'n' close to zero yield "extended" structures (p is maximized), whereas higher values of 'n' produce structures with a more compact design (p is minimized).

Common repeating secondary structures are listed in Table 6.1.

Non-repeating secondary structures for a polypeptide are also common. These include structures called"random coils" because they lack a repeating structure.