Stopcock (main water shut-off) & isolation valves

A stopcock is the main valve that turns off the water supply to your home — the one you shut in a burst or major leak to stop the flooding. Alongside it, most fixtures have smaller isolation valves (under sinks, basins and toilets, and on the water-heater feed) that let you cut the water to just that fixture — so you can change a tap cartridge or fix a leaking joint without draining the whole house. The single most useful thing here is simply knowing where your main stopcock is and that it actually turns *before* an emergency: in a burst pipe, the minutes spent hunting for it are the minutes your floor floods. Isolation valves seize up with hard-water scale, so it's worth checking they still operate. Where a valve is seized, corroded or the shut-off won't hold, that's a licensed plumber's job to free or replace — ideally before you actually need it in a hurry.