Originally Posted by
icemilkcoffee
First of all, the OP is correct in referring to 26" tubular tires as 26". They were always labeled as 26" by tire manufacturers.
Secondly, 26" tubulars are nominally same diameter as 650c, which is 571 ETRTO.
Except when not, as shown in my post above. 26" can also refer to ISO 559 for mountain bike usage. In typical road usage yes 26" is likely ISO 571 at least in this context but some companies have made ISO 559 road bikes. Though tubulars wouldn't be super common for that as they were generally made for consumers and not professional racers who would be likely to use tubulars.
Also we don't know what the OP is referring to as they haven't really been clear yes they could actually be referring to sew ups but they aren't clear about which ISO they need as technically there are 7 different 26" sizes according to
Sheldon Brown . However lets say even in the case of actual sew ups we have two sizes to contend with because they didn't list road or mountain and just weren't clear.