Those lines marked with numbers that go across your ruler are for cutting 60 degree (or 30 degree) angles – especially equilateral (60 degree) triangles and diamonds. These are used to make hexagon shapes.
All rulers have these lines – some have more than others and some are clearer than others.
First cut your strip. For equilateral triangles cut the strip 3/4 (three quarters) inch wider than the required height of the triangle. So if the finished size of the triangle is 3 inches, cut a 3 + 3/4 inch wide strip.
Trim one end of the strip to a 60 degree angle. Which end depends on whether you are right or left handed among other things – dance around the strip and cutting mat until you decide which end feels most comfortable. Find the 60 degree line on the ruler and place it on one of the long sides of the strip –
Now turn the ruler so it faces the other way, place a 60 degree line on a long side of the strip and the edge of the ruler against the tip of the first cut. This can sometimes involve a lot of dancing round the mat and turning the ruler over and round, but eventually you will find the right edges to line up (and it is much easier in the privacy of your own home than in a classroom full of students – or ‘sewing in public’ as one of our students described the feeling).
To cut the next triangle you need to dance around the mat again to line the 60 degree line up with one edge of the strip and the ruler going the other way to meet the tip of the triangle you just cut –
Diamonds involve slightly less dancing! Thiis time add 1/2 inch to the width of the strip for a seam allowance – for a 2 inch wide diamond, cut a 2 + 1/2 inch wide strip. Trim the end of the strip as before.