A guest ordered ten replacement bulbs for a downlight fitting and every single one arrived with the wrong base, because "the old one" wasn't a GU10 at all, it was a bayonet cap that only looked similar in the photo.
GU10 has two pins and a quarter-twist locking action, used almost exclusively in downlights and spotlights. Bayonet (B22) has two side pins that push and twist into a socket, common in older pendant and lamp fittings. Edison screw (E27 standard, E14 small) threads in like a bottle cap. They are not interchangeable, and getting the wrong one means the bulb simply won't fit the holder.
How Do You Tell GU10, Bayonet and Edison Screw Apart?
The confusion almost always comes from photos. On a screen, a GU10's two short pins and a bayonet's two side pins can look similar enough to order the wrong one, especially when the old bulb has already been thrown away and there's nothing to hold up for comparison.
| Fitting | How It Connects | Where You'll Usually Find It |
|---|---|---|
| GU10 | Two short pins, push in and twist a quarter turn to lock | Ceiling downlights, spotlights, kitchen and bathroom recessed fittings |
| Bayonet (B22) | Two pins on opposite sides of the base, push in and twist to lock | Older pendant lights, table lamps, many UK ceiling roses |
| Edison Screw (E27) | Threaded metal base, screws in clockwise like a bottle cap | Table lamps, decorative fittings, many European-style fixtures |
| Small Edison Screw (E14) | Smaller threaded base, same screw action as E27 | Candle-style bulbs, chandeliers, smaller decorative fittings |
Why Doesn't a GU10 Just Fit a Bayonet Socket?
The two pins on a GU10 base are spaced differently from a bayonet's, and a GU10 locks with a quarter twist into a ceramic or plastic holder shaped specifically for it. A bayonet base pushes into a spring-loaded socket instead. Neither the pin spacing nor the locking mechanism is compatible with the other, so there's no way to force a GU10 into a bayonet holder or vice versa, even with adapters, which don't exist for this particular mismatch the way they do between screw sizes.
The standard for modern downlights. If the fitting is recessed into a ceiling or a spotlight housing, it's almost certainly this one.
Common in UK homes built or fitted out before LED downlights became standard. Still widely used in table lamps and pendants.
E27 is the full-size Edison screw, E14 the smaller version for candle bulbs. Common in decorative and European-style fittings.
Does the Fitting Type Affect Dimmer Compatibility?
The base type itself, GU10, bayonet or Edison screw, has no bearing on whether an LED bulb dimmers properly. That depends entirely on whether the bulb is rated dimmable and whether the dimmer switch is rated for LED loads, most commonly a trailing edge dimmer. A dimmable GU10 LED and a dimmable E27 LED both need the same compatible dimmer type; the connector shape has nothing to do with it.
Where fitting type does matter is stock and replacement speed. GU10 is by far the most commonly stocked LED format for downlights, so if a property has mixed lighting, some GU10 downlights and some bayonet pendants, it's worth keeping both types on hand rather than assuming one will do for both. For anyone speccing lighting at volume rather than replacing a single bulb, it's also worth reading why Luceco has become the trade default for commercial LED lighting, since fitting type is only one of several specification details that catch out a first-time buyer.
Not sure which fitting your downlight takes? Kent Traders stocks GU10, B22 bayonet and E27/E14 Edison screw LED bulbs, dimmable and non-dimmable, with bulk discounts from 2 units.
Shop LED Bulbs by Fitting →What Should You Check Before Ordering Replacement Bulbs?
- Check the base on the actual old bulb, not a memory of what it looked like or a photo taken from a distance.
- Count the pins or look for a thread: two close-set pins is GU10, two wider-set pins is bayonet, a threaded collar is Edison screw.
- Note the wattage and colour temperature printed on the old bulb if you want a like-for-like replacement rather than just a fitting match.
- Confirm dimmable vs non-dimmable separately from the fitting type; both are printed on the packaging but answer different questions.
Frequently Asked Questions
Get the Right Bulb, First Time
GU10, B22 bayonet and E27/E14 Edison screw LED bulbs in stock. Dimmable and non-dimmable. Bulk pricing from 2 units.
Shop LED Bulbs →
