Of course, everyone knows where the actual baseline is on the doubles court. However, in a doubles match, it is smart to never aim at the actual baseline.
 
If your opponent is behind the actual baseline pretend you are aiming at an imaginary line half way between the baseline and the service line, commonly called 3/4 depth. This allows you to over hit and still get your shot in and still keep your opponent back. However, as your opponent moves forward (inside the baseline) toward the net, it is smart to imagine that the baseline is always right in front of your opponent close to their feet. When you see the court this way you quit making errors "long" over the baseline and "wide" across the sideline.
 
You will start playing higher percentage tennis. You will reduce your long and wide errors, forcing your opponent to hit up on the ball, protecting your partner. Remember... keep hitting the ball "under" your opponent or at their feet. It will prevent many unforced errors and protect your partner. The baseline is always moving as your opponent moves. Pay attention to where your opponent is on the court.
 
Good luck on the courts!