In my experience, having lived in both Germany and the US, they are not the
same. They are a millimeter different.
E27 bulbs (German) worked fine in our E26 (US) fixtures. But the reverse w
as not always true. I have a couple of German lamps that do not reliably t
urn on with a US bulb. Sometimes US bulbs fit, sometimes not. The manufac
turing variation is apparently enough to allow some but not all US E26 bulb
s to work in E27 fixtures. I do not recall ever having an E27 bulb not wor
k in an E26 fixture.
Yes, I know bulb is an incorrect term, but I stayed with the terms of this
And yes, I know voltage is different, but that's easily solved.