Find your keys; square, flat, gib-head, round...

[Toolmaker51]

Keys, along with keyways and keyseats, offer dependable driving connections between shafts and hubs.
Sensibly, key size is related to shaft diameters, those combinations are typically based on power to be transmitted. A secure connection occurs when shaft and hub diameters fit to engineered tolerances. A close fit reduces shearing action across the key, through key seat and key way depths, and key seat length, to a degree.
Those fits and depths are easily referenced via Machinist's Handbook, many manufacturers, and scores of web pages.
So, to energize this thread, I've prepared an illustration of various keys, along with comments.