It's a common question among golfers, because no matter where you place that rake - inside or outside the bunker - it will still be in a position to influence golf balls rolling its way.
So what's the rule? Well, there is no rule, which, of course, is what leads to the confusion. Although in Decision Misc./2 (see the Miscellaneous Decisions section of The Rules of Golf and the Decisions on the Rules of Golf on usga.com), the USGA does state, "Ultimately, it is a matter for the Committee to decide where it wishes rakes to be placed."
It's likely that your golf club or course has such a decision in place, so the first thing to do is to ask the golf course for its policy on rake placement. If they have one, then simply follow that policy.
And if the course does not have a policy, or you are unable to find anyone who knows what it is? While there aren't any official rules about rake placement, there are rules of thumb and guidelines provided by the USGA in Decision Misc./2.
The USGA states in that decision:
"There is not a perfect answer for the position of rakes, but on balance it is felt there is less likelihood of an advantage or disadvantage to the player if rakes are placed outside of bunkers."
True, a rake placed outside a bunker might cause a ball to careen into the bunker, while a rake that is already in the bunker might cause a ball to careen out of the bunker.
More likely, though, when a rake inside a bunker influences the ball, is the possibility that the ball will come to rest against (or even on top of, if its teeth are pointing up) the rake.
Continuing with Decision Misc./2:
"It may be argued that there is more likelihood of a ball being deflected into or kept out of a bunker if the rake is placed outside the bunker. It could also be argued that if the rake is in the bunker it is most unlikely that the ball will be deflected out of the bunker.
"However, in practice, players who leave rakes in bunkers frequently leave them at the side which tends to stop a ball rolling into the flat part of the bunker, resulting in a much more difficult shot than would otherwise have been the case. This is most prevalent at a course where the bunkers are small. When the ball comes to rest on or against a rake in the bunker and the player must proceed under Rule 24-1, it may not be possible to replace the ball on the same spot or find a spot in the bunker which is not nearer the hole - see Decision 20-3d/2."
But what about placing the rakes in the middle of the bunker, where they won't be able to stop a ball on the bunker's sloping sides?
"If rakes are left in the middle of the bunker the only way to position them is to throw them into the bunker and this causes damage to the surface. Also, if a rake is in the middle of a large bunker it is either not used or the player is obliged to rake a large area of the bunker resulting in unnecessary delay.
"Therefore, after considering all these aspects, it is recommended that rakes should be left outside bunkers in areas where they are least likely to affect the movement of the ball."
The Golf Course Superintendents Association of America further recommends that rakes outside the bunker be placed laying flat on the ground (tines up) and parallel to the hole's direction of play.
So: Follow the guidelines in place at the golf course or in place for your tournament. If such guidelines are not in place, or you are unable to learn what they are, then place rakes outside bunkers, parallel to the direction of play on that hole.
We've seen several golf courses addressing the question of rake placement in creative ways, ways that make the question moot.
At one such course, tubes were sunk into the ground outside its bunkers, and the rake was dropped down into the tube with the rake head laying flat against the turf.
At another such course, rakes were attached to golf carts, rather than left next to every bunker. (Such a course would have to prevent walking, however, or put up with walkers being unable to rake bunkers.)