The mortise & tenon was the basic woodworking joint used before the advent of modern fasteners -- and since. It has been used in everything from barn building to fine furniture. If you have ever tried to make this matched pair by hand you might have been frustrated by simply not having the right chisel.
The tenon is the easiest part to make but it should not be cut first. The mortise is the hardest to get to a precise dimension, so it is always made first and the tenon is then shaved to fit -- rather than expecting them both to be perfect. This is an absolute necessity when working by hand and even a good practice when working with machinery.
When working with machine set-ups, make all the mortises the same way, then adjust the cutting of one tenon until it fits and make all the others the same. When working by hand, size each tenon to each corresponding mortise.
Chiselling out mortises is one of those workshop tasks that is long and slow, but for me has always been one of the most enjoyable parts of woodworking as you get into a drumming kind of rhythm with a good chisel and a good mallet.
The real key is to have two specialized chisels. A mortising chisel has fully square sides and you work with one that is exactly the width of the mortise you are making. The square sides of the chisel actually guide the chisel through the wood. The curved mortising chisel is used to work deeper down.In the fourth photo you can see how it works with the handle much higher, allowing you to work deeper.
The DIY technique for making mortises is to first drill out most of the hole and only use the chisel for cleaning off the sides. In this application, two regular carpenter chisels work better than the real mortising chisel, one slightly less than the width of the mortise and the second at least as wide as half the length of the mortise. These chisels work better for smoothing off sides than the real mortising chisel which is designed to remove massive material. If you try using them as you do a tight fitting mortising chisel, they will just get jammed -- with carpenter chisels you need room to slice from side to side.
Today's professional woodworker has no time for such pleasures as hand made mortises, so they use the special square mortising chisel/drill. The square housing has four chisel faces for slicing down into the wood, and a spiral drill in the centre for cutting out the meat and lifting the chips up and out. Basically it is doing both steps of the DIY technique in one pass.
A mortising machine is a specialized machine with sliding guides and controls for making rapid repetitive mortises. There exists a semi-pro version of the mortising machine which uses the same chisel/drill sets locked into a special collar designed as an accessory for your drill press.
Just getting back into the rhythm of the chisel and mallet during this episode made me nostalgic for my old cabinet shop in the days before Mr. Chips drug me in front of the TV cameras. Whenever I could steal the time, I would make them by hand.