The term core set is especially applicable to soft ferrite cores, some of which comprise several parts, usually two or three, but in special cases there can be more.
Single-part (like toroids) do not require additional parts to form the complete set. Many core types (like ETD) are designed to be made from two identical halves (Fig. 1). However, in the case of a gapped core (Fig. 2) it is possible to form a core set from one ungapped and one gapped half - because the gap is present only in the central cylindrical part.
Some core types (e.g. FRM20) are designed such that exactly defined two parts must be used (Fig. 3, Fig. 4).
In some cases the parts can be used in alternative ways to form a fully functional core sets. For instance, the EI cores (Fig. 5) can be composed either as E+I or E+E.
Three and more components can be used in core sets like RM or pot cores (Fig. 6). The main magnetic path is created by joining the two large core halves, and the central aperture is designed to accept and additional “adjuster” which can be used for fine tuning the required level of inductance.2)