Most of the mosaic tesserae on the market today (about 99.99%) are regular, square cut pieces. Every side being either 10mm or 20mm. These can be used for making a Roman mosaic but they present a very uniform appearance.

 

On straight lines obviously they are fast to set and you can easily regulate the size of the gaps, but you end up with a very rigid style.

On curved lines it's a little different as with cutting you can lose some of the rigid shapes. The downside of this though is that you need to do a lot more cutting than if you were using irregular sized tesserae and this increases the time your work takes.

 

The image below shows a couple of shapes I set using square tesserae.

 A. The curved line, I've used keystone shaped cuts to get the line of the curve without having too bigger gaps on the outside edge of the tesserae. On each curve though I've had to cut at least 3 keystones and this is too many. In a Roman mosaic if a keystone is used it's only about 1 on each curve. Using irregular shaped pieces you can match up the edges to enable you to follow the curve without resorting to more cutting.

 

B. With the horizontal lines, seen here at B and on the off white area above I've made some half cuts to off set the tesserae slightly and also varied the angle of some of the pieces.

 

In summary, you use square cut tesserae to make a Roman mosaic but be prepared to do a lot more cutting to 'soften the look of your finished piece.