Original outlines in φ,ψ space proposed by G.N. Ramachandran in 1963.  Solid lines enclose region allowed by hard-sphere bumps at standard radii; dashed lines show region allowed with reduced radii; dotted lines add region allowed when the tau angle (N-Calpha-C) is relaxed slightly. (Note that Ramachandran called the angles φ and φ', and that he put 0°,0° in the lower left corner rather than in the center as here - standard now since 1970.) Labels show the approximate location of common structural features such as α or 3-10 helix.
<a href="//commons.wikimedia.org/wiki/User:Dcrjsr" title="User:Dcrjsr">Dcrjsr</a>
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<span class="int-own-work" lang="en">Own work</span>