This Staedtler Reduction Scale Ruler is a great tool when you're doing technical drawings. It allows you to create accurate lines at an appropriate and consistent scale.
The ruler itself is 30 cm long so you can easily keep it handy.
It allows you to create at scales of 1:20, 1:25, 1:33, 1:50, 1:75 and 1:100.
It has a durable plastic construction to withstand frequent use.