A person designated and raised to become the next king.
A person expected to inherit or take over an enterprise or undertaking once its current manager retires or dies.
A person designated and raised to become the next emperor.
A person who rules or governs; someone or something that exercises dominion or controlling power over others.
A (usually rigid), flat, rectangular measuring or drawing device with graduations in units of measurement; a straightedge with markings.
To beat with a ruler (as a traditional school punishment).