Ok just when I thought I knew everything. From knowing that a light bulb shape such as E26 is the common lightbulb base (mean Edison screw type with is 26mm wide) also know and medium to the light bulb shape (an A19 which is the familiar “normal light bulb) A shape.
Then we got into the more exotic sizes like E11 which is 11mm wide and the incompatible E12 which is or course 12mm.
These are usually in T for Tubular shapes which can be T4 for 4mm long.
But every so often even know these, you get a strange one. In an outdoor lighting fixture there is a bulb with a unique bayonet with two contacts on either side. It’s a low voltage connector called S8.
I only found it by looking at them on a site and seeing a huge collection of automotive light bulbs. They are often called 3156 bulbs and are used for backup, brake and center mounts. Note there is a 3157 which is dual filament and brighter