A comparative analysis of the main-chain conformation of the L1, L2, L3, H1 and 
H2 hypervariable regions in 17 immunoglobulin structures that have been 
accurately determined at high resolution is described. This involves 79 
hypervariable regions in all. We also analysed a part of the H3 region in 12 of 
the 15 VH domains considered here. On the basis of the residues at key sites the 
79 hypervariable regions can be assigned to one of 18 different canonical 
structures. We show that 71 of these hypervariable regions have a conformation 
that is very close to what can be defined as a "standard" conformation of each 
canonical structure. These standard conformations are described in detail. The 
other eight hypervariable regions have small deviations from the standard 
conformations that, in six cases, involve only the rotation of a single peptide 
group. Most H3 hypervariable regions have the same conformation in the part that 
is close to the framework and the details of this conformation are also 
described here.