This shows how to set out a geometric Roman mosaic border pattern, double strand guilloche, in a room ensuring the pattern turns correctly. I have scaled this down to show the work but this works on any size.
This illustrates how these border patterns need to turn at a set point and very rarely turn exactly where you want them to.
What to do if your room isn't square will be covered in another article.

1. Using a staff mark the width of your pattern.

corner staff 1

2. Next mark the distance between the centre tesserae (CT).

corner staff 2

3. I've taken a random measurement and drawn in the area in which the mosaic is to be set. The cross hatched area on the outside represents the wall of the room.

corner staff 3

4. Using either a protractor or set square mark a line at 45 degrees from each corner into the centre.

corner staff 4

5. lay the CT mark on the staff on this line with the width mark on your outside (wall) line and mark. This shows the closest the pattern can be set to the wall.

corner staff 5

6. If you lay the staff, parallel to the outer line on the 45 degree lines with one point (B) on the line you can see that at the other end (A) the CT mark does not sit on the 45 degree line. If you tried to set the border on this line then it wouldn't turn the corner properly.
It this case you have to move the staff, always keeping it parallel to the outer line, further into the centre of the mosaic area.

corner staff 6

7. Move the staff in until there is a CT mark on both 45 degree lines but ensuring that the distance is the same at C and D. Mark this point on both lines. This is the closest the border pattern can go to the outer edge or wall and still be able to turn correctly.

corner staff 7

8. With the CT mark of your staff on its mark on the 45 degree line, mark the outer width mark (E). Do this on either side of the 45 degree line all the way around, 8 marks in a square.

corner staff 8

9. Draw in a line connecting these (E) marks which creates the outer line of your border pattern which is marked F here. 
Place the staff with the outer width mark on F and the CT mark on the staff should line up with H, the CT mark on your 45 degree line. Move the staff along until the inner width mark on your staff meets the 45 degree line, I, and mark. Do this on all your 45 degree lines and draw in the line.
Then draw in another line connecting up the CT points you've marked on the 45 degree line.
G shows the mark done originally in picture 5 to show the outer limit for the CT line.

corner staff 9

10. Here you can see all the lines drawn in.

corner staff 10

11. On your CT line using the staff mark in your CT points.

corner staff 11

12. Here is the pattern drawn in. The full drawing of the pattern is fairly rough, it's important to remember 2 things; guidelines are just that, guidelines. You only use them to get a rough idea of your pattern and you can adjust this as you set your tesserae. Also you're using tesserae which are slightly irregular in size so you can either make up space or 'pinch' lines as you need to. How much you can get away with is a matter of experience.

corner staff 12

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