>>2942766
The one not shown is where the biscuit idea came from- splines.

Not just strong, but extremely versatile- you can use them to locate/reinforce many other types of joints and in different orientations, they can run the entire length of where boards meet or their grooves can stop short to create a "blind" joint that is essentially a blind loose tenon joint except tenons are usually far thicker and bear loads where a spline or biscuit acts like a fastener and leaves more surface area for glue to do the work.

If you use quality plywood for the spline the way biscuits do you can orient it any way you want, solid wood splines need to orient the grain so the forces affecting the joint can't break them along the weak grain direction.
You can also make them from metal or FRP for applications where that makes sense, but one benefit they share with biscuits and dowels is you can cut/drill/plane/turn through them in subsequent operations and not ruin your tools...and unlike those others the spline runs along the entire joint so you can do stuff like make a box or octagon shape and then cut it into smaller sections to make identical parts like shallow drawers in a jewelry box or small frames.