28" | 700A | 700B

There are 4 different 28" tyre sizes, which match with 4 different 700 sizes (700, 700A, 700B & 700C). 

700C tyres are your commonly used 700x23, 700 x 35 etc tyres.

The only reliable way to match up a 28" tyre is by using the ETRTO number (European Tyre & Rim Technical Organisation). The first number refers to the width, and is not critical. The second number refers to the bead seat diameter, and this must be matched accurately. This number is usually on the side of the tyre and looks like, say 37-642.

French Size  ERTRO   Inches

700A              28-642    28 x 1-5/8 x 1-1/2

                       37-642     28 x 1 3/8

700B              40-635    28 x 1 1/2