The mass of an object as measured in its own frame of reference is called its rest mass or invariant mass and is sometimes written m 0 . If an object moves with velocity v in some other reference frame, the quantity m = γ γ ( v ) m 0 is often called the object's "relativistic mass" in that frame. Some authors use m to denote rest mass, but for the sake of clarity this article will follow the convention of using m for relativistic mass and m 0 for rest mass.