According to lighting industry standards, E26 and E39 are U.S.-standard lamp sockets that comply with the UL 496 screw-type socket standard; whereas E27 and E40 are European-standard lamp sockets that comply with the IEC 60238 screw-type socket standard. In practice, there are slight differences in the thread dimensions between E26 and E27, and between E39 and E40, which can be measured using standard gauges.
