One at the wall and one at the spout, and the pair exists because of where the tap is. The spout valve is the one used in service; the body valve isolates the arm so a drip or a repair does not mean shutting off the kitchen supply.

It also gives a second line of defence over an appliance. Water released unnoticed above a hob runs onto a heat source and down cabinetry, and there is no basin beneath to catch it.

The two are closed in sequence: the spout valve stops the flow, the body valve empties the arm. Leaving the arm charged is what produces the drip that appears hours later.

Almost always cold only. The pot is going to be heated anyway, so hot water buys nothing at the tap and would require a second supply run to a position that has no drainage.

There is a hygiene dimension as well. A hot supply drawn infrequently to a remote outlet is a stagnation risk, and a pot filler is by nature used in bursts rather than continuously.

Single-supply also keeps the articulated arm simple. One waterway through the joints is easier to seal reliably than two, and every joint in a folding arm is a potential leak above an appliance.

Enough to place the spout over the largest burner, which for a domestic range usually means reaching the far side of the hob from the wall behind it, and rather more on a commercial range.

Reach is delivered by the joints rather than by one long spout, so the arm folds back clear of the cooking zone when not in use. That folded depth is the other dimension to check, since an arm that protrudes is in the way of pan handles.

Deck-mounted versions exist for islands where there is no wall behind the hob, and they need a longer vertical riser to clear tall stockpots.

It needs to be assessed for one, because the outlet can sit below the rim of a filled pot, which is a submerged-outlet risk rather than the free air gap a sink tap enjoys.

Whether a device is required, and what type, depends on the local water regulations and on the fluid category assigned to a cooking vessel, which differs between jurisdictions.

The practical answer on installation is to establish the requirement before first fix, because retrofitting a backflow device into a wall-mounted arm means reopening the wall.

Water runs onto the hob and behind it. Unlike a sink tap, there is no basin and no drain, so a slow leak collects on an appliance and travels into the cabinetry and the wall build-up.

This is the reason the body valve exists and the reason it is worth using rather than relying on the spout valve alone. An isolated arm cannot drip.

It is also why pot fillers are specified with quarter-turn ceramic disc valves rather than washered ones. Ceramic discs either seal or fail obviously, whereas a perishing washer weeps for months.

Only with a supply run to the wall behind the hob, which usually means opening the splashback and the wall, and that is the whole cost of the job.

Where the hob backs onto an accessible void, a cupboard or a stud wall, the run is straightforward. Where it backs onto solid masonry with tiling over it, the work is substantial and best done during a refit.

Deck-mounted models are the retrofit alternative on an island with existing plumbing below, though they need a hole through the worktop beside the hob and still require an isolating valve within reach.

One that tolerates repeated cleaning of cooking residue and the heat and steam rising off the hob. Solid stainless and good-quality chrome plate both cope well.

Living and unlacquered finishes such as raw brass and copper will patinate faster here than anywhere else in the kitchen, which is fine if that is the intent and disappointing if it is not.

Matt and powder-coated finishes are increasingly popular and vary considerably in how they respond to degreasing cleaners, so the maintenance guidance matters more on this tap than on one over a sink.