Up to this point the call number system makes straightforward, alphanumeric sense. But it is when we add the second part of the call number that things become more complex. This is because the second part of the call number takes on a decimal value. Thus:

F102 comes before F30 which comes before F9, because .102 is less than .3 which is less than .9

Remember: The second part of a call number is a decimal!