Most of our materials are organized on the shelf using Library of Congress call numbers. When looking at the number on the book spine, these would be read one line at a time.
Books are arranged in alphabetical order, by the letters on the first line of the call number.
For example, first come all the D call numbers, then all the DA call numbers, then DB, etc.
Within the DA call numbers, books are arranged in number order.
The numbers are arranged in numerical order, from low to high.
Line 3 of the call number has a letter and a number. The letters are in alphabetical order.
Once you find the letter, read the number. But note these are decimals not whole numbers.
Example: A55 is read as A .55. This is why A55 comes before A6 (A .55, A .6, A .65).