Full Idea
Relativity has two components: to the choice of a background theory, and to the choice of how to translate the object theory into the background theory.
Gist of Idea
Two things are relative - the background theory, and translating the object theory into the background theory
Source
Willard Quine (Ontological Relativity [1968], p.67)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.67