To read a call number, look at each part separately. Read from left to right and top to bottom. Let's look at the number E645 .H37 2021 for the book titled: Monumental Harm: Reckoning with Jim Crow Era Confederate Monuments by Hartley.
Books are arranged in alphabetical order from the first letter(s) on the first line of the call number.
E would come before EA; ES would come after EA and before EZ.
After the letter(s), books are ordered numerically. Numbers are treated as whole numbers. 645 is read as a whole number. For example, 645 numerically comes after 641 and before 675.
This starts with a letter organized in alphabetical order, followed by a number read as a decimal, NOT as a whole number.
So .H37 will come after H211 (since .211 becomes before .37) and before H4 (since .37 comes before .4).
There may be additional lines read in the same way.
The last line will be the date of publication.
What Call Numbers Mean
The Library of Congress Classification (LCC) system has 21 classes, each identified by a letter of the alphabet. Sub-classes are identified by a combination of two or three letters. Topics within sub-classes (e.g.place or time period) are broken down into one or more numbers.