The 26 is the measurement of the diameter of the tire. The number 1.75 is the width of the tire. Occasionally the second number will be represented in fractions.
Some companies will measure the diameter from the lip of the tire across the middle, some will measure from the bottom of the first tread. This leads to differences when buying new tires.
Some companies will measure the width from across the lip of the tire. Other companies will measure across the outside of the tire on the treads.
A tire that is marked with a 1 3/4 marking will not fit in a 1.75 wheel. This is an inconsistency between companies. Usually a single company's tire will fit a wheel of the same size.
International Organization for Standardization (ISO)
ISO is an organization trying to apply a universal measurement to bicycle tires (among other products) across the world by using millimeters as the standard unit. This will make things easier for the consumer to purchase the correct products for their bike.
Article Written By Heather Broeker
Originally from North Carolina, Heather Broeker studied journalism and advertising at the University of North Carolina. After graduation she moved to Los Angeles, where she worked for Fox Searchlight, Fox Reality and later as a writer and marketing director. Broeker now lives in Los Angeles and runs Head Over Heels, a writing and public relations company.