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.

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.

This is the big brother of the Quicker Trimmer. It helps in the same manner to trim 20 cm blocks and measures 21,5 x 21,5 cm. Divisions are four centimeters.

This ruler is only available as long stocks last. It will not be replaced.