Define primary structure

The primary structure (also called the released or basic determinate structure) is the statically determinate and stable structure obtained from an indeterminate structure by removing the redundant forces, that is, redundant reactions or internal forces such as a support, a member, or a moment restraint. The removed redundants are then applied as unknown loads and found from compatibility conditions.

A structure with $D_s$ degrees of indeterminacy needs $D_s$ releases. The primary structure must remain stable, and it is not unique. Example: for a propped cantilever, the primary structure may be the cantilever (prop removed) or a simply supported beam (fixed-end moment released). In the force method every analysis starts by choosing it, finding $M_0$ for the loads and $m_i$ for unit redundants.