How Capslock works: F18, an event tap, and a mapping it gives back

Fri, 06 Nov 2026 00:00:00 +0000

Caps Lock sits in the best spot on the keyboard and does the least useful job. Capslock gives it a new one: tap it for Escape, hold it for Hyper, and combine it with the four modifiers to reach sixteen control planes. This post explains how a native app does that on macOS without a kernel extension, a driver, or a privileged helper.

Caps Lock is not a modifier

macOS does not report Caps Lock as a key that goes down and comes back up. It reports a lock state that flips, and it adds a short delay so that brushing the key does not toggle it. That is fine for typing capitals and useless for a modifier that must be held.

Engineering on Capslock

How Capslock works: F18, an event tap, and a mapping it gives back

Caps Lock is not a modifier