A5 Bullet Journal Ruler
It's a straight edge, a ruler, a column and row guide, a bookmark, and likely to be one of the most useful planner accessories you own. It is made of flexible, durable, plastic, with (new version released!) 5mm increments printed down the left edge that perfectly align with a standard 5mm grid and a standard cm/mm rule down the right edge. Works great with gridded Moleskine, Scribbles That Matter (STM), and Leuchtturm1917 squared A5 journals and more!
The Journal Rule provides a total box count at a glance. Divide your page into 2, 3, 4, 5, 6, or 7 equal columns or rows.