Ever wondered why there's a line of white tiles (tesserae) around figures in a Roman Mosaic? Look at any Roman figural mosaic, whether it be one found in Britain, Spain, Italy, North Africa or the Middle East and you will see the figures out there.
This is just one of what I would term 'The Roman Rules', methods of setting the tesserae which if you know them you can see in every mosaic across the Roman Empire. If you do not know them then the chances are you would not notice them at all. This is what differentiates Roman mosaics from modern mosaic work. In modern works they follow no rules, these pieces can not be criticized. What a lot of contemporary artists do not know is that the visual effects that they are trying to create can be dependent on the use or non-use of these ancient rules.
Why did they have them?
Roman floor mosaics were designed to be seen as a whole, what the mosaicicsts did not want was for there to be any point of focus. One area of the floor which draws the eye in every time the room is entered. You may have central panels which have quite fine figural work but the idea is for all the lines of tesserae to 'flow' To give you an extreme example, imagine you have a floor, say 5m x 5m. The floor has had a mosaic laid and this has been done using tesserae which are about 10mm cubes, so that's about 260,000 tesserae. Now imagine that the mosaic has been laid using only white tesserae, all in horizontal lines.
Each of these horizontal lines needs to occupy its own space, ie each line starts on one side and continues straight across to the other side. If the mosaicist did not keep the lines straight and 2 lines ended up blending into 1 line then where this occurred on the floor it creates a point of focus. Once you have noticed this then every time you enter that room your eye would be drawn to that spot. Obviously this is an extreme example but these rules were applied all the time. Next time you see a Roman mosaic of a figure look for the borderline, if it's not there then it's a mistake, or a very bad copy.