Today I'm thinking about the best way to stack boards (to support extensions in other words). In my search I stumbled upon these DIP adapters

I find Arduino shields bulky. And they can only grow upwards. I want to have flexibility in this regard.

There's such technique as "castellated vias" where connections are on the edge of the board. Like through holes that metallized inside but located on the edge and cut in half

First I shook off this idea, because it's used mainly for adding daughter boards, so I was considering pinheads. But they restrict the direction of attachments—it only goes in the way it's designed to go.

But then I revisited initial limitations, and soldering wasn't in the list. The credit goes to Jeremy Cook and his LED cubes

I'm thinking about an extension board for 2 more bi-directional motors. I put one full row of vias on the edge and get castellated edge (I do the bottom row as well). OSH Park can manufacture such edge but with possible imperfections