It was somewhere outside Barstow when Jim Martin
wrote:

Is there a corner joint for boxes, drawers, etc that is made with three
pieces: the two parts plus something like a loose tenon?

That would be a spline. If you allow "2+n" parts, where "n" can be
large, then you can use a keyed joint. Splines are a single wafer
running along the joint, keys are lots of little wafers running across
it.

I don't like splines in mitred corners - there are short-grain
weaknesses.

Keys come in two main flavours; thick and thin. Thick are big thick
things, deliberately visible. They're often made to use "dovetail"
shaped keystock, which needs to be made separately. If only one side
of the joint is visible (e.g. drawers with false drawer fronts applied
afterwards) then these can be indistinguishable from dovetails.

Thin keys are parallel and the thickness of a saw kerf (although this
can be quite a thick blade, or even a dado). Their advantage is that
the stock is easy to make and the joint is cut on a single pass. If
you do these in two batches with a tilted sawblade, then they can have
an attractive staggered look that's reminiscent of dovetailing.
They're _very_ quick to make and you can use them to strengthen a
joint you've already made.

Both versions need a jig to cut them. The jig for the first is usually
a small table to support a sliding router, this table having a pair of
45° fences underneath to support it across the corner. Use a dovetail
cutter and slide it through the edges in one pass. Saw the key stock
up on the tablesaw, but be careful to get the width exactly right.
Leave the height over-tall, you'll plane the excess off later.

To make the thin sort, make a sliding sled for your tablesaw slots.
More 45° fences support the box above it.

As with all contrast timber techniques, go easy on the ebony and maple
look. Fumed or ebonised oak in white oak looks much better.

--
If we fail, then let us fail heroically
(or even better, stoichiometrically)