What is the difference between E39, E40 and EX39 mogul base

E39 E40 and EX39 are all called mogul base.

What is the difference between them?

Well, E39 E40 are almost the same, they are compatible with each other, just the E39 is for North American market and E40 are for Europe market. 

While EX39, the diameter of the base is the same with E39 E40, but the tip on the base is longer, so their socket is different in depth. Therefore, they cannot be compatible with each other’s screw socket. 

If you buy mogul base wrongly and the cost to replace a correct one is expensive, then you’d better to buy a E39 to EX39 adapter