On Fri, 13 Oct 2017 11:44:02 GMT, Sarah
m wrote:

replying to clare, Sarah wrote:
E26 (US) are one millimeter shorter and may not make contact with the base.
No contact, no light. Longer, E27 bulbs will make contact in the shorter
base. I have had the same experience. All my USA purchased lamps worked
great in Europe. None of my European lamps or ceiling lights worked in the
USA without bulb base adaptors. It has been so long since I purchased them I
can't tell you where to go.
The 26 and 27 refer to the thread diameter, not length. An E26 will
thread all the way into the larger E27 base with no problem, while the
larger E27 bulb MAY not thread all the way into the 1mm smaller
(nominal) E26 base.

In practice, the precision ov the thread form is so poor there is
virtually no perceptible difference in MOST sockets or bulbs. All the
Chinese crap is likely E26.5 with a tolerance of +.5 /-0 on the
sockets and +0/-.5 on the bulbs.