φ,ψ plot ("Ramachandran diagram") of the torsion angles that specify protein backbone conformation, with the alpha-helix and beta-strand regions labeled.  Data as in Lovell et al. 2003 Proteins 50:437, showing about 100,000 data points for general amino-acid types (not Gly, Pro, or pre-Pro) in high-resolution crystal structures.
Dcrjsr
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>