The best way to put together a mortise and tenon is by drawboring them. What this means is that when you drill the holes for a dowel to pin the joint together you offset the hole in the tenon towards the shoulder so that the pin has to bend a little to get the joint together and put a little tension in the joint. Any shrinkage in the stile would be taken up by the tension and the joint is incredibly strong.
The problem is test fitting it. You can of course just drive dowels into the joint but that means driving them out and replacing them every time you test fit. This is a pain and also wears the joint. The traditional solution is using drawbore pins. Drawbore pins not only let you assemble the joint but they have an eccentricity that allows you to tighten up the joint without banging down. It's a technique that is still used today to align steel beams for assembly.
A few years ago I mentioned to Ray Iles that there were no proper drawbore pins being made on the market and certainly none with the proper eccentricity. He mentioned that he would get antiques in and they would sell very quickly. He took a long look at a bunch and saw the eccentricity that I mentioned and of course he said "that explains why you don't see hammer marks on them." Most of the old ones were used by doormakers and large rigging makers but and they work for cabinetmaking (it's what I have) but smaller sizes can be much easier to handle.
The way you use them is just put them in a drawbore hole and wiggle them down. You will notice that on half a turn they seem to tighten up the joint, and on the other half they loosen. Just by turning and wiggling the pin you can get the joint to mechanically tighten up with a lot of pressure. A pin without the offset, is sometimes used but you don't get the pressure on the joint.
While real drawbore pins like these for woodworking haven't been made since around World War One. You can buy modern drawbore pins for steel work from any good supply company (see photo at right). These are short and meant to be turned with a large wrench but they do the same thing. What steelworkers use them for is pulling beams together into alignment so the beams can be bolted or riveted together. The important thing is that, just like our drawbore pins, you have loads of leverage from the eccentricity in the pin.
These are not an essential tool for the occasional mortiser but for anyone doing large mortises, and/or lots of mortises, these drawbore pins will enable you to test fit joints much faster, much tighter, and save a lot of time.
Sold in two sizes, the size reflected the diameter of the shaft of the pin above the taper. Typically you would be using a dowel size smaller than the pin size so that your registration happens somewhere on the tapered part of the drawbore pin. Due to the colossal leverage they exert they work better on hardwood than softwood where the holes can more easily distort. They are typically used in pairs so that you can assemble one entire side of a frame and panel when checking for fit and square.
Made in England, offset, hardened, with steel bolsters, and a beech handle.