Mammalian immunoglobulin VH families can be grouped into three distinct clans based upon sequence conservation in two of the three framework (FR) intervals. Through replacement/silent site substitution analysis, molecular modeling and mathematical evaluation of known immunoglobulin crystal structures, we demonstrate that this conservation reflects preservation of protein sequence and structure. Each clan contains a characteristic FR 1 interval that is solvent‐exposed and structurally separated from the antigen binding site. Families within a clan contain their own unique FR 3 interval that is capable of either influencing the conformation of the antigen binding site or interacting directly with antigen. Our results provide a structural context for theories that address differential use of VH families in the immune response.