What is a foot fault in tennis?
During the service motion the server must stay behind the baseline, between the imaginary extensions of the centre mark and the sideline, and may not change position by walking or running. Touching the line or the court before contact is a foot fault.
A foot fault is treated like any other fault: on a first serve the server gets a second, on a second serve it's a double fault.
Questions
Can your foot be in the air over the line?
Yes. It's only a foot fault if a foot touches the baseline or the court inside it before you hit the ball.
Is a foot fault a double fault?
Only on a second serve. On a first serve it's a single fault.
Sources
- ITF Rules of Tennis