The difference between 75a and 75b lies in the size of the cup. 75 is the lower bust, and ab is the cup. 75a is the upper bust of about 85cm, 75B is the upper bust of about 87.5cm, and the B cup is bigger than the A cup. The difference between cups is 2.5 cm. A cup represents the difference between upper and lower bust 10 cm, B cup represents the difference between upper and lower bust 12 cm, and C cup represents the difference between upper and lower bust 15 cm. You can choose the right underwear according to the difference of your upper and lower bust.