My solution is as brilliant as it is simple:

A square 16,5 x 16,5 cm ruler.
For use with rotary cutters of course.

Exact, with a clear layout and easy to use.

See at a glance where your seamlines will be.
Helps to avoid cutting off the tips of your patterns.

Has few lines to show center point and diagonals exactly.
Useful for geometric patterns.

New! Now including lines for 10 and 5 centimeter blocks. Useful too for trimming quick triangle blocks to size.

This way to the tip on paper pieced quick triangles.

The nine patch crosshairs make horizontal and vertical alignment easy.

Know the Rule of Thirds? As in photography you may use it for fuzzy cutting.
A motif placed on one of the four intersecting cross lines will always create a pleasing layout.

Lines are visible even on black fabric.

Last but not least: A hole for hanging.