The 559 mm (26") size fits mountain bikes and cruisers.
The 1- 1.25 width fits the 26 x 1.0 or 1.3 tire size.
The 1.5- 1.75 fits the 26 x 1.4 to 1.75 tire size.
The 1.75- 1.95 fits the 26 x 1.75 to 1.95 tire size.
The 1.9-2.125 width tube is the standard size for tires marked 26" x 1.9", 2.0", or 2.125".
Since tubes are stretchy, they do not need to exactly match the size of the tire.
Depending on stock, size shipped may not be exactly the same as listed in the pull-down menu.
Make sure you specify valve type:
Schrader valves are the same as used on automobiles.
Presta or "French" valves are narrower, require a different pump.
I cant believe that tube manufacturers say the small diameter tubes (26 x 1.9-2.125) fit cruiser tires (2.125), they do not. Do yourself a favor and order some bigger tubes (26 x 2.1-2.125) or fight blowouts constantly.
