This is my favourite tool of the moment. It's a graders set square, and it's an essential tool for pattern drafting. It could be used for any type of pattern, not just dressmaking, and it would also be handy for adjusting ready printed patterns. Having only had this for a couple of weeks, I would now be quite lost without it!
On one of the short sides it has lines which are spaced at 0.5cm intervals. This is handy for measuring seam allowances - either adding or subtracting them.
On the other shorter side of the triangle is a ruler of 25cm with a line exactly 1cm from the edge of the rule.
On the long edge is a 'double ruler' which starts in the middle and extends either side to 20cm. This is also underlined with a line exactly 1cm from the edge.
The double ruler is particularly handy for measuring darts either side of a flare line.
Straight down the centre of the triangle is a printed straight line. This can be aligned on vertical or horizontal lines to ensure the correct anglular positioning of other lines.
By placing the set square on a designated line and lining any given point up with the other lines or squares printed on the set sqaure, you can get perfect 90 degree angles and you can measure a required amount either side of a vertical or horizontal line.
By using the line in the middle and aligning it appropriately, you can draw a perfect cross grain line. This is handy when you want to cut a fabric on the bias.
The set square is particularly wonderful when used in partnership with Dot and Cross paper
The paper is marked with alternate dots and crosses spaced at 1cm intervals. The vertical lines represent the straight grain of the fabric (parallel to the selvedge), and the horizontal line represents the warp threads. You can determine the exact cross grain of the fabric by joining the dots in a diagonal line.
For this and other sewing supplies, check out the Morplan website.