regul

The root regul begins with a very concrete object: a straightedge. Latin rēgula was the carpenter's or mason's straight stick — the tool you laid against a board to check whether it was true. That stick came from the verb regere, "to guide in a straight line, to keep straight," which also meant "to govern" (a ruler keeps a kingdom straight, just as a ruler keeps a line straight).

From that one image — measuring against a fixed standard — the whole family grows:

- A board that matches the straightedge is regular: even, consistent, conforming to the rule. Over time "matching the standard" softened into "happening at fixed intervals" (a regular train) and "ordinary" (a regular guy).
- A board that does not match is irregular (ir- = not): uneven, breaking the pattern.
- To regulate (-ate = to make) is to keep something lined up with the rule: governments regulate markets, a thermostat regulates temperature — both are holding a system to a standard.
- A regulation is the written standard itself — the official rule you must measure up to. Regulatory describes the whole apparatus of governing-by-rules.

The most everyday member took a detour. Rēgula passed through Old French reule and arrived in English as rule — and here the two halves of regere fused: a rule is both a standard ("the rules of the game") and the act of governing ("under British rule"). The plastic measuring stick you used in school is also called a ruler, preserving the literal sense, while a king who rules preserves the governing sense.

One native-feeling word hides the root: unruly = un- (not) + rule + -y — literally "not able to be ruled." An unruly crowd, unruly hair: things that refuse to stay in line.

regere seeded a wider clan beyond regul. It is the same source as rect ("straight, right": direct, correct, erect — directing or making straight), and as reg/rex ("king, ruler": regal, regent, regime — the one who governs). So direct, regular, and regal are cousins, all tracing back to the idea of keeping things straight and in order.