The Singles Game
8. Foot Fault The Server shall throughout the delivery of the Service: a. Not change his position by walking or running. The Server shall not by slight movement of the feet which do not materially affect the location originally taken up by him, be deemed "to change his position by walking or running.
b. Not touch, with either foot, any area other than that behind the base-line within the imaginary extensions of the centre-mark and side-lines.
Source:Tennis Online
That IS a foot ON THE LINE, no?
