Full Idea
We let 'R' be the accessibility relation: xRy is read 'y is accessible from x'.
Clarification
The R is usually in a script font
Gist of Idea
We let 'R' be the accessibility relation: xRy is read 'y is accessible from x'
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.5)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.9