Fleißner grille
A rotating grille cipher that reads the plaintext through holes in a template turned four times.
The Fleißner grille is a transposition cipher. A template with holes is laid over a square grid, the plaintext is written through the holes, and the template is turned a quarter turn between passes so that every cell is covered once. What remains is a square filled with letters in an order that looks random to anyone without the template.
It refines the older Cardano grille, which had only one orientation. Rotation makes it harder to reconstruct, but the principle is the same: whatever shows through the holes is the message, and everything else is padding.