Piano keys are wooden levers usually cut out in order like jigsaw puzzle pieces from a wide panel or keyplank.
Keyplanks are glued up to the width required out of a few boards with the grain going the long direction of the keys. The wood they're made of has to be dry, straight grained and free of knots, as well as stable with changes in humidity and relatively strong and easy to work. They're usually a light weight, soft wood with little difference between early and late wood, like soft white pine
. English piano engineer Samuel Wolfenden
considered pine a little too soft, and like the famous German piano manufacturer Julius Blüthner
considered basswood a little too unstable, depending on where it grew. Different woods can be combined to make better advantage of their good qualities.
The keys shouldn't bend so they should be relatively thick, depending on the amount of space there is, but usually they aren't much more than 2.5cm or less than 1.5cm. Shorter keys can be made thinner, and thinner keys can also be made stiffer by increasing the height just around the fulcrum, using stiffer wood or by reducing the stuff they have to move.