It's not too pretty, but here is my prototype:
|The very first Beginner's Tool Kit Square.|
I chose lap joints because my basic tool kit doesn't have a chisel narrower than 3/8" yet. The lap joint should be plenty strong for this. Besides, if it comes apart, you can easily make another one.
Construction is pretty straight forward, just lap joints and perhaps some chamfering if you so desire. The idea is that the handle is thicker than the blade, so you can push the handle up to your true edge and mark with your pencil or knife along the blade.
In retrospect, I recommend not going chamfer-happy like I did. You'll notice that there is a nice pretty chamfer that makes it comfy to hold like a pistol (indeed, when I showed it to my wife, she shot me with it). However, it may reduce its accuracy if it slips on the work you are trying to mark square.
I also don't think having the joint off-set looks as nice here as it might on one with a bridle joint. Not to fear, this only took a couple hours to make, mostly because I was being anal about squaring my stock.
Stay tuned for Mark II.
Music: No Money No Honey All We Got Is Us (The Underscore Orkestra) / CC BY-NC-SA 3.0