L1 bulbs have the same electrical connector as GU10 bulbs, but are only used on low energy bulbs.

The difference is that there is a hole in the center of the bulb between the pins. This is for a non electric pin in the lamp holder to slot into. The extra pin in an L1 lamp holder prevents the use of inefficient halogen lamps while allowing the use of L1 bulbs in GU10 lamp holders.

They are often found in newer houses built when building regulations required the use of dedicated low energy bulbs and in all light fittings from tp24.

The L1 lamp cap and lamp holder is a design from tp24.