(The term frame is used in a different sense in modal logic than in geometric logic where it stands for an abstraction of the algebraic structure corresponding to the lattice of open sets of a topological space; see frame.)
We will start with the simplest case, namely a frame for the basic modal language, , (so with one unary modal operator, denoted ).
A frame for is a pair with a non-empty set and a binary relation on .
(This is also called a basic Kripke frame.)
The terminology often used refers to as the set of possible worlds. Its elements are sometimes called worlds, sometimes states, sometimes points, depending on the context and the whim of the writer. The relation is called the accessibility relation so says ’ is accessible from ’.
(N.B. Here we are still restricting to multimodal logics in which the modal operators are unary. The generalisation to allowing more general -ary modalities will be considered later.)
The generalisation is not difficult. In , the single unary modality is modeled by one binary relation. In , there are -unary modalities so the frames have -binary relations. Explicitly we have
A frame for is a -tuple with a non-empty set and for each , a binary relation on .
(More to go here … frames with unary and more general relations.)
To give the standard (geometric) semantics of modal logics one needs the models and these will be discussed in the entry geometric models for modal logics and the companion algebraic models for modal logics.
Generally this entry is based on
(any mistakes or errors of interpretation are due to ….!)