A call number is the code that appears on the bottom of the spine on a library book. Cerro Coso uses the Library of Congress classification system, which determines where the book is located on the shelf. 

Let's take a call number example: QE534.2.B78 1999

To find this on the shelf, it's easier to break it down alphabetically, numerically, by decimal, and then by date.

QE
534.2
.B78
1999

Start alphabetically (A, B, C, ...Q). Once you hit the Qs, start over again alphabetically (Q, QA, QB, QC, QD, QE). Once you are at QE, start counting numerically (1, 2, 3, 4, 5 ... 100 ... 500 ... 534, 534.1, 534.2). Treat the decimal in these numbers as you would in regular math. If it helps, think of 534 as "534.0."

The alphanumerical combo (library nerds call this a Cutter number) in the next part can be a little tricky because this number is treated like a decimal - notice the period before the B? This means that .B78 will come before .B9, in the same way that 1.78 is numerically smaller than 1.9. Who knew there was so much math in library science?

There is sometimes a second cutter number which does not have a decimal, but should be treated the same - alphabetical, then numberical. 

Curious what the letters and numbers actually mean?

These are all parts of the Library of Congress classification system. Let's break it down to see what each piece means.

QE  - The first letter is the general subject division. In this case, "Q" is Science. The second letter is a subdivision. In this case, "E" is Geology. There are always 1-3 letters.
534.2 - This number tells us more about the book's subject. The range QE 500-625 is about "Dynamic and Structural Geology." Books with the number QE534.2 are specifically about "Earthquakes, Seismology - General Works - 1970 to Present." 
.B78 - The Cutter number is a coded representation of the author or organization's name. In this case, the .B78 is for the author David S. Brumbaugh. If there are two cutter numbers, the first is a further breakdown of the numbers above and the second is the author's name.
1999 - This is the year of publication. It's especially helpful when there are multiple editions of the same book (usually a textbook).