Good doubles teams know the value of covering the center of the court.
"Cover your line!" is screamed so often in doubles. When does this make
sense to a doubles team?
Think about the ball as it crosses the net during the point. On a majority
of shots you will find that the ball crosses the net near the center strap.
The serve crosses near the center if it is to be in, the return is usually
crosscourt to avoid the server's partner at the net, and the first volley
will travel across the center strap more times than not. Of course there are
the occasional returns down the line and the high volley at the receiver's
partner, but you will find that smart doubles teams play consistently to the
area they make fewest mistakes.
Remember that one of the characteristics of a solid team is a low number of
unforced errors. These teams know that they need to cover the center of the
court because it is the highest percentage shot for the opponent. Angles
are still possible by going over the center strap but if you aren't close to
the net, extreme angles shouldn't be attempted.
Always cover and anticipate to the middle of the court with one exception:
when the ball is wide on your side, cover your alley and let your partner
move to the center. If you move with your shots you will naturally be
covering the middle of the court as well as the alley. The less experienced
team will cover the alley and the lower percentage shots and make it easy for
the opponent to hit the easiest shot - down the middle. If your opponent is
at the baseline and you cover the center of the court, you force them to hit
to you, hit a low percentage sharp angle, or hit down the line over the
highest part of the net. Make them hit for the edge of the court.
So when in doubt on court coverage, make sure that the middle of the court is
covered. You'll find that you are more prepared for a majority of the shots
and you will probably force the opponent into the lower percentage shot.