Keystoning is a cutting technique in which your square tesserae goes around curves without leaving a large 'V' gap on the outside of the curve. When executed well your grout lines will run parallel and you won’t have a triangular space between the tiles. This technique has been practiced since ancient times, as you can see in the mosaic below.
So why use keystoning? When you use this technique your tiles follow a continual curve, and fit together from top to bottom. The tiles maintain a parallel edge so the grout lines are all consistent.
This is what you are trying to avoid:
Here is a technique I developed when first practising keystoning, with a bit of practice however you will be able to get the correct angle just by sight.
Find the centre of the circle or curve
Position the tile so that the maximum surface area is against the edge of the circle or curve.
Get a ruler and position it so that it runs from the centre of the circle or curve to the tile.
Draw a line on the tile where the ruler runs, this is your cutting line. You are basically removing that part of the tile that doesn't follow the edge of the circle or curve.
Here are a few key points about keystoning:
The smaller the curve the sharper the angle.
You should find that your tiles are about the same size and shape when cutting tiles for a circle (you may need to make an adjustment to the last tile to fit the circle).
If you make an angle too sharp the next cut will have to be cut on the opposite angle, which will result in the circle or curve losing its flow.
If you find it difficult to achieve the right angle make small cuts and work the tesserae to the right shape slowly.