You might cheat a little you your left, to protect your weak backhand, or, if you are much stronger than your partner, you might try to cover more than half the court, but mostly, you and your partner will line up at the X's in the diagram above.
This is elementary, and quite obvious. Unfortunately, it is also very wrong. The experts know this, even though the rest of us don't. Watch some videos from recent Nationals. There are points where top players like Dan Gabanek and Jagan Subhas plant themselves virtually on top of the center line, leaving their entire side of the court unprotected. Why? What do they know that we don't? Simple, they know the Fifteen Foot Rule:
Only guard fifteen feet of the volley line.
This is surprising, and a bit counter-intuitive, so I am going to spend some time explaining why the rule works.
The Geometry of the Game
Let's go back to our example. After your deep-middle return, your opponents have a couple of options. They can go soft or hard. Soft means lobbing the ball or dinking it into the kitchen. Hard means hitting a low, sharp passing shot.
If they go soft, it doesn't matter where you line up. You will have plenty of time to chase down a soft lob or dink. Indeed, you could be off enjoying a cup of coffee at Starbuck's and still have time to chase down a dink.
It's when they go hard that you need to be ready, and in the right position. This is really a fundamental principle of play that applies to many situations:
Position yourself for the hard shot.
You should always be ready, and in position, for a hard hit. If they dink around you or lob over your head, chase it down.
All right. Now let's imagine that your opponent hits a good passing shot that gets by you and your partner. Where will it land?
Deep! This is simple physics. A hard low shot will rise up you the net on their side and drop down on yours, travelling virtually the same distance on your side of the net as on theirs. Good top-spin will drop it down a bit faster, but, for most players, a passing shot hit from the baseline will land within half-a-dozen feet of your baseline.
Here is a diagram of the possible trajectories of these hard passing shots:
These shots trace out a triangle that is 38 feet long, with a 20 foot base. You and partner are at the volley line, which is 29 feet away. By similar triangles (math alert! Remember those?), that triangle intersects the volley line in a segment that is 15 feet wide (actually just a touch over 15'3").
Only guard fifteen feet of the volley line.
You don't have to worry about the two-and-a-half feet near each sideline. If your opponent hits there, it is either a soft shot, and you will get to it, or it is hard, and going out. You are left with seven-and-a-half feet of court on your side that you have to protect, not ten feet. You should position yourself right smack in the middle of that 7 1/2 foot zone. This will put you fifteen inches closer to the center line than the traditional line-up of Figure 1.
Let's try a second example: Again you are on the left side, receiving serve. This time you put the return deep and cross-court, into the server's backhand corner (another nice shot!). Where do you line up?
Again, there is only 15 feet of volley line to protect, but, in this scenario, your opponent's hitting triangle opens up from the far right corner. The 15 feet starts at the right sideline. Both you and your partner should shift two-and-a-half feet over to the right (thirty inches, or one full stride). You will end up one foot from the center line, with your right foot on the line, or slightly across.
It was these positions that looked so strange in the videos from the Nationals. Players moved all the way over to the center tee, apparently daring their opponent to hit into the open court, because the return had gone to the far corner. They lined up, correctly, in the Z-positions above.
To recap: Don't try to guard the entire volley line. Line up to protect a fifteen foot stretch. Your initial ready-position is at Y, but you move to the right or left if the ball is to the right or left of center.
Here is a third example: Again you are on the left, receiving serve. This time you try that new fancy slice return you've been practicing, but, you mis-time it, and it lands right at midcourt, and pops up. Where do you go?
This is much different. A strong opponent can hit that type of "sitter" with pace to virtually any area from the volley line back. Forget 15 feet. You have to guard the entire volley line. To make matters worse, you will probably have to volley a low shot, which is very hard, and, with the ball at midcourt, you'll have less time to react.
Personally, I would stay deep and try to give myself more room to defend. Whatever you do, you are in trouble, and rate to lose the point.
This is an important example. The receiving side has a big advantage in pickleball, provided they return deep. That advantage shifts on a short return, and the geometry leading to the Fifteen Foot Rule vanishes. As a matter of personal strategy, if my opponent returns the serve deep, I usually try to dink the ball, and bring the point back to neutral. If the return is short, I go for the kill.
The Fifteen Foot Rule in the Dinking Game
The Fifteen Foot Rule seems to apply on any deep ball, but disappear on a short ball. It must follow, then, that the Rule doesn't apply once we transition to a dinking contest. After all, an opponent can dink the ball virtually anywhere across the kitchen. You and partner must guard all twenty feet. True, yet, oddly enough, the Rule still applies.
Let's analyze. Suppose your side has dinked the ball very near the center line, and your opponent is about to dink back, as in this diagram:
A dink back to the center will drop quickly, and you had better be in range. As you can see from Figure 5, that dink will only travel a few feet, leaving little time to react. A dink to the sideline, however, travels three or four times as far, giving you plenty of time to move to such a shot and play it back. So, you want to line up closer to the ball. Our usual Y-ready positions are excellent ready positions for dinking.
More importantly, many dinking bouts end when a dink gets too high, and someone punches it back hard. A hard volley like that, hit at a very sharp angle, will go out. You have to guard all twenty feet of the kitchen for dinks, but much less for hard volleys.
Even in a dinking game, the usual principles apply, and we should position ourselves for the hard shot.
So, pinch in closer to partner that you used to. Line up at the Y-ready position after returning serve. If you start dinking, keep to that position, and follow the ball. If the dink lands near their far sideline, slide over to the center line, position Z, and trust partner to slide over to cover that sideline. Likewise, if the dink is near your sideline, move over to cover a sharp tap down the line. Leave the center to partner.