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.
Here are a few key points about keystoning:
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