The short answer

A 1-way light switch controls one light from one switch position. It has two terminals: incoming live (L or COM) and switched live to the lamp (L1).

A 2-way light switch controls one light from one switch position, but is wired so it can be combined with a second 2-way switch to give two-position control of the same light (one switch at the top of the stairs and one at the bottom, for example). It has three terminals: common (COM) and two strappers (L1 and L2).

Almost every modern UK light switch sold today is a 2-way switch, even if you only need single-position control. The extra terminal simply stays unused.

Terminal count: the easy way to tell them apart

Take the faceplate off and count the terminals on the back:

• 1-way: 2 terminals, usually labelled L and L1 (or COM and L1).
• 2-way: 3 terminals, labelled COM, L1, and L2.

Some UK switch ranges (especially newer ones) ship every switch as a 2-way with three terminals – you simply ignore L2 if you only need 1-way operation. This is convenient for stock-keeping but means visually you can’t always tell which is which without checking the terminals.

How a 1-way switch works

A 1-way switch is a simple single-pole single-throw (SPST) mechanical contact:

• Switch on: contact closed between L and L1, current flows to the lamp.
• Switch off: contact open, no current to the lamp.

The internal mechanism is a single brass rocker that either bridges or breaks the connection between two terminals. Nothing more.

How a 2-way switch works

A 2-way switch is a single-pole double-throw (SPDT) contact:

• One position: COM connects to L1, breaks contact with L2.
• Other position: COM connects to L2, breaks contact with L1.

There is never a position where COM is connected to neither L1 nor L2 (apart from the brief instant of the mechanical switching action). The internal rocker always rests against one of the two strapper terminals.

This is the basis of a UK 2-way light circuit: two SPDT switches connected through their L1 and L2 strappers, so flipping either switch changes the contact path and toggles the light on or off regardless of the other switch’s position. See our full 2-way wiring guide for the complete circuit and diagram.

When to use each

Situation	Use
Single switch position controlling one light	1-way (or 2-way ignoring L2)
Two switch positions controlling one light (stairs, hallway, large bedroom)	2-way at both positions
Three or more switch positions controlling one light	2-way at the ends, intermediate switches in the middle – see the intermediate switch guide
Two independent lights from one switch position	2-gang 1-way (each gang is a separate 1-way switch in the same plate) – see the 2-gang switch guide
One light with dimming control from one position	1-way dimmer – see the dimmer wiring guide
One light with dimming from one position and on/off from another	2-way dimmer + 2-way switch – see the switch-to-dimmer guide

Can a 2-way switch be used as a 1-way?

Yes – this is extremely common in UK installations. Wire COM as the incoming permanent live and L1 as the switched live to the lamp, and simply leave L2 disconnected. The switch will operate exactly like a 1-way:

• Position one: COM connects to L1, light on.
• Position two: COM connects to L2, no circuit (since L2 isn’t wired anywhere).

The unused L2 terminal stays empty. Some installers insulate it with a small length of brown identification sleeving for tidiness, but it is not required.

Can a 1-way switch be used as a 2-way?

No. A genuine 1-way switch only has two terminals and no L2 – there is nowhere to connect the second strapper. If you need to convert a single switch position to a 2-way circuit (add a second switch elsewhere), you need to replace both the existing switch and the wiring back to that switch. See our conversion guide.

Why most UK switches sold today are 2-way

It is simpler for manufacturers to make all switches as 2-way SPDT with three terminals – one design, one product, fits every installation. Retailers stock one SKU instead of separate 1-way and 2-way variants. The cost difference is negligible (a couple of pennies of extra brass and one extra terminal screw).

The practical implication: when you buy a “1-way light switch” at a UK builders’ merchant or DIY store, you are almost always getting a 2-way switch with three terminals, intended to be used as a 1-way by ignoring L2. This is normal and entirely safe.

Identifying terminal markings on UK switches

UK switches use a few common terminal markings:

• L or COM or C: the common (incoming live in a 2-way, or simply the live in a 1-way).
• L1: first strapper terminal (or simply the switched live in a 1-way).
• L2: second strapper terminal (2-way only).

Older UK switches sometimes used different markings (such as Top/Bottom, or numbers 1-2-3). If in doubt, a multimeter on continuity mode will tell you which terminals connect in each switch position.

Frequently asked questions

I have an existing 2-way switch but only want it to control the light from one position. Do I need to change it?

No. Use COM as your incoming live and L1 (or L2) as your switched live to the lamp. Leave the other strapper terminal unconnected. The switch will function as a 1-way without any modification.

If I add a second switch to a 1-way circuit, can I keep using the original switch?

Only if it has three terminals (in which case it is a 2-way being used as a 1-way). A genuine 2-terminal 1-way switch must be replaced with a 2-way to support second-position control.

What cable do I need for 2-way switching?

Three-core-and-earth (6243Y) between the two switch positions. Twin-and-earth (6242Y) is sufficient between the consumer unit and the first switch, and between the second switch and the lamp. See our UK lighting cable colours guide for the full conductor identification rules.

Can I wire a dimmer as a 2-way switch?

Most modern UK dimmers, including the Samotech SM323 range, support 2-way operation. Wire one position with the dimmer (using COM and L1) and the other position with a standard 2-way switch. See our switch-to-dimmer wiring guide.

How can I tell if my existing switch is wired as 1-way or 2-way?

Isolate the circuit, remove the faceplate, and count the cables entering the back-box. A 1-way installation has one twin-and-earth cable; a 2-way installation has one twin-and-earth at each end plus a three-core-and-earth running between the two switches.