I have a screw thread lightbulb where the thread is 11.5 mm across - would that be likely to be E11 or E12? Or something else? Thanks.
The E26 and E27 are usually interchangeable, as are the E39 and E40, because there is only a 1 mm difference in thread outside diameter. E11 and E12 are not interchangeable.
The E11 base is sometimes used for 50/75/100-watt halogen lights in North America, where it is called the "mini-can", and tighter threads are used to keep them out of E12-base nightlights and other places where they could start a fire.
Thank you, Wikipedia.
So E11 is a thing, it is used for high intensity halogens. That bulb is a common night light or festoon (Christmas style string) light, so it will be E12.
Presumably E12 since it rounds to 12 mm and E12 is a more common size. See this: https://www.12vmonster.com/blogs/product-questions/edison-screw-types