7/8". aka 22.2mm only MTB/straight bars and , now swept back flat bars work well , an angled grip design new this year.
http://www.ergon-bike.com/us/en/product/gc1

but others have gotten along adequately fitting them to the back of their Trekking bars , also 7/8" tube.

Had a Mk2 Brompton , they used a 15/16" kind of tube used , it was a no-go, new ones are 7/8, so Ergon grips fits OK
[I use shorter grip shift versions , and augment them to fill in shorter space with a wrap of bar tape. )

With the M bar I'd have to cut down their full length grip.