What is keystoning? Keystoning is a cutting technique used on square tesserae being laid along a curve, that eliminates the problem "v" shaped gaps which otherwise occur like this:
When you keystone your tesserae, they follow a continual curve, fitting very close to each other, from top to bottom of each tile.
How long has keystoning been practiced? If you look back at the ancient mosaics, you will see keystoned tesserae on all the curved areas in these mosaics. This technique has been around since mosaics got their start.
Why do you need to learn the keystone technique for mosaics? Keystoned tesserae maintain parallel edged grout lines running perpendicular to a curve.
What cuts do you need so your tiles go around a curve or circle? The objective is to eliminate "V" shaped gaps so that each tesserae meets at the top and bottom corners. Your first tile may be a square tile that begins the curve. The second tile may be cut slightly at an angle on both sides, for more gradual curves. You may have several cuts exactly the same way, it depends on the size of your curves. The basic rule is: the tighter the circle, the sharper the angles must be cut.
The main thing to mastering this technique is practice. Try practicing the various cuts until you get the hang of it. Below are some videos that demonstrate in great detail all you need to know to master keystoning.