Why is there a groove on the back of window trim?

Why is there a wide shallow groove on the backside of window trim?

It’s so that if the back of the trim has to be taken down a bit to make the trim sit in the correct plane, you only have to take down (with a block plane, usually) the high spot on one side of the trim, not across the whole back. This description may be a bit opaque, but imagine that the surface to which the trim is applied is not 100% planar. Suppose that the left window jamb sits a bit high relative to the adjacent plaster wall surface. The right edge of the left-hand piece of window trim then needs to be reduced a bit so the trim will sit flush against the plaster, and not twist, which would affect the fit with the top piece of window trim. This reduction is easier if you only have to plane a way the section that is higher than the groove, rather than plane away across the entire back.

If this is still confusing, I’ll try to come up with a diagram.

Sal Ammoniac Perfectly clear answer. Thanks. This has been bugging me for a while.

If you are talking about casing - interior trim for windows and doors, the groove is called a rebate. It is also common with baseboard.

To add to what Sal has said, the casing bridges two surfaces, drywall and the window frame or door jamb. The rebate basically helps to keep the the center of the casing out of the way, the edges of the casing are the part that makes contact and is glued and nailed.

Ideally the drywall and jamb or frame edge should be flush, but due to the inexact nature of frame construction this often isn’t the case. Quite often the cut edge of the drywall will also be rough or flared. In both cases you can end up with a gap between the casing and the surface it is supposed to mate with. The rebate helps reduce the necessity of using a plane or other means of fixing the situation.

Thanks to Roy Underhill’s wonderful show, The Woodwright’s Shop, I just learned that rebate was the way rabbet was spelled in England about 180 years ago, and pronounced more like rabbet than the money you get back from buying something overpriced in the first place.