Light bulbs in the New York City subway system screw in “backwards” (i.e. with left-handed threads) so people won’t steal them to use at home.

Edison screw

Three-way E26d light socket

E26 Edison screw to NEMA 1-15 adapter

Specifications for all lamp mount types are defined in the following American National Standards Institute (ANSI) and International Electrotechnical Commission (IEC) publications:

Lamp Caps—ANSI C81.61 and IEC 60061-1

Lamp Holders—NSI C81.62 and IEC 60061-2

Gauges (to ensure interchangeability)—ANSI C81.63 and IEC 60061-3

Guidelines for Electrical Lamp Bases, Lampholders and Gauges—ANSI C81.64 and IEC 60061-4

Generally, the two standards are harmonized, although several types of screw mount are still defined in only one standard.

In the designation “Exx”, “E” stands for “Edison” and “xx” indicates the diameter in millimeters as measured ac… Continue Reading
Source: https://en.wikipedia.org/wiki/Edison_screw#Types