Question on Schwalbe Tube Sizes for Brompton

[RhythMick]

I just bought two Schwalbe AV4 inners from Wiggle, site says they are for Brompton and they would seem to match. I use the Marathon Greenbelts.

Confusingly though, both the box and the inner tube say "16/18". How can an inner be both 16 and 18 inches ? I'm missing something obvious and will soon be bowing my head in shame I'm sure.

Specifically, the tube says...

Schwalbe
No 4
16/18" - 28/37 340/355

 

[StuAff]

I was curious, never heard of a tube suitable for different diameters before either, so I had a look. According to Schwalbe's own tube listings, the AV4s are compatible with 340, 349 & 355 sizes (from 37-340 to 35-355).
You're fine, basically.

 

[RhythMick]

Under ETRTO: on the tyre it says...
28/37 - 340/355

Just can't get my head around how the same tube could fit both a 16" wheel and an 18"

 

[RhythMick]

Yep ta.