This paper examines the steering inputs that cause rear-wheel skid. First, we compare the front and rear side force at maximum lateral acceleration and thus distinguish rear-wheel skid from front-wheel skid. Next, we examine the affect of forward velocity on rear-wheel skid, and show that rear-wheel skid cannot occur below a given forward velocity. We use this velocity as an index of rear-wheel skid tendency and formulate it. This formula shows that the difference between front and rear axle trajectories strongly affects rear-wheel skid.