Evenly

your eye already knows even.

drag beads along a slack cord until every gap between them matches

There is no ruler and no number here, just gaps that look right or do not, and the instant they match, your eye settles before you could explain why. Getting it even is the whole payoff — there is no lesson hiding underneath the feeling of it.

Concepts: spacing, symmetry.

An interactive you play with one thumb, on Wundr.