Q: Positions during serve- When you are serving, must your partner be behind the back line? Is it a rule? Thanks :)

A: When serving, your partner should be back with you…behind the line, but not required, and it is not a rule, but it is a strategic and wise choice to be back and is a defensive move.

The reason being:
The opponents will almost always want to return the ball deep, they want to keep you deep and they do not want your team (serving team) to get to the net. The main goal is to return the serve deep to keep the serving team at the baseline; power is not as important as control. A short return brings the serving team forward, allowing them to reach the non-volley zone (kitchen) line and negating the receiving team’s advantage. 

The net is where points are won. If your partner is inside the base line (server must be behind the base line). That becomes a target for your opponents. Hit the ball hard and deep to that person (your partner) because your partner will be out of position when the ball is hit at their feet, and the serving team MUST LET THE BALL BOUNCE – definitely compromising your position when accepting the return.

Additionally, you and your partner should always be in concert together. As if you were both holding on to a broomstick. You need to work together, move together forward and back, and left to right. You are forming a wall fighting your opponents.