Using space-filling models, G. N. Ramachandran [Adv. Protein Chem., 1968, 23, 283 explored the sterically accessible conformation space of some simple peptides.

• He described his results in the form of what is now called a Ramachandran diagram, a graph of y against f.

• Molecular dynamics simulations more recently have been applied to refine the contours [Anderson and Hermans, Proteins, 1988, 3, 262; Scully and Hermans, J. Mol. Biol., 1994, 235, 682].
  

• The medium blue in this diagram represents regions of "no steric hindrance"

• The lighter blue areas are "allowed", but have "some hindrance".

• The white areas are the "badlands" - disfavored for most amino acids

• Positions corresponding to some typical protein secondary structures are indicated, and also tabulated below

These ranges are not absolute, nor are the outlying territories "forbidden" in any sense.

• We should regard the colored areas as "basins", within which rotation is relatively easy

• The white areas are simply "badlands", a little more difficult to explore

The graph below is a Ramachandran plot for the enzyme chorismate mutase, prepared using VMD.

• The yellow squares are the data points; you will notice that many of them fall outside the "allowed" contours.

• Some of these correspond to sharp turns in the chain.