My DeWALT DW682K Plate Joiner is one of my favorite tools. It makes precision slots for easily aligning pieces prior to glue application and clamping.
Slots are cut into adjoining surface with the joiner. Side-to-side positioning is easily achieved by aligning the red line on the joiner with a straight line drawn across pieces. Consistent depth is achieved by using the tool on consistent sides (ie, placing the tool on the outside board for a miter cut on both pieces will ensure the miter aligns).
Once the slots are in place, a “biscuit” wedge is glued and inserted into the slots for a tight fit. Of course it’s not really a “rubber biscuit“. But I’m still nostalgically using the canisters I bought years ago at Sears. The biscuits align the pieces, provide a broad surface for glue, and they add strength to the joint. You can even get a slot glue bottle, but I found the glue to dry-out too quickly and not add much value.
With the biscuits in place you can even dry-assemble furniture to make sure it’s all coming together. I’m even using them on 45-degree miter cuts to ensure alignment.
Some assorted photos:
WidgetWood 