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One of the most important skills you'll learn is reading by the Library of Congress standard. This is an academic method of shelving library books, and isn't unlike the Dewey Decimal system you may be familiar with. Books are grouped by subjects, with letters assigned different meanings. It may seem intimidating at first, but it's straightforward once you get used to it.
Here's a video from the UAA Library Resources Center that helps explain shelving.
While shelving, follow the call number from start to finish, and place materials in alphabetical and chronological order.
Start with the letter. These mark a subject, and should be grouped together. For example, "A" goes first, followed by "AB" and then "AX".
Next, read the second line as a whole number, and place in order accordingly.
Read the third line's letter first, then the number.
Some call numbers might have a fourth line: read these, and place them in order after the third line.
Finally, many call numbers will include the year of publication. Place these in chronological order.