Structural domains (the units of fold) are independently stable tertiary structures of proteins (Figure 16). They are distinct functional and/or structural units and can evolve, exist and function independently. Therefore, the same domain can be a part of different proteins.

Figure 16 Three domains of Thermus aquaticus elongation factor EF-Tu: in blue (all-β), red (α/β) and green (all-β).

You can learn more about domains in the introduction to protein classification course.