As the name suggests, a square head stopcock key is for turning on and off mains water stopcocks that have square heads as opposed to T-shaped, or crutch, heads.
The square recess in the key head fits over the square head of the stopcock. Turning the key anti-clockwise turns the water supply on, and turning it clockwise turns it off.
If your stopcock looks like this, you need a square stopcock key. However, most modern stopcocks have crutch heads, which are like traditional T-shaped taps
The square head stopcock key is an ‘outdoors’ tool – that is, it is used on stopcocks located outside, usually below ground level, under a metal or plastic cover.
What sizes do they fit?
A basic square head stopcock key fits just one size of stopcock.
Square head stopcock sizes are 3/8 inch (10mm), 1/2 inch (15mm), 5/8 inch (16mm), 3/4 inch (20mm) and 1 inch (25mm), with 1/2 inch (15mm) and 3/4 inch (20mm) being the most common. The size relates to the diameter of the pipework attached to the stopcock.
There are square head keys to fit all of these stopcock sizes, and many fit more than one size.
The handle of a square head stopcock is usually welded to the shaft, which can be round or square section.
The handle is often flattened at one end to form a kind of chisel or point. This can be useful for prising up the metal covers over outdoor stopcocks and removing any debris or mud from around the edges of the cover.
Square head stopcock keys have long shafts so that they can reach stopcocks that are sometimes several feet below ground level.
They are usually between 860 and 1060mm (34 and 42 inches) long.