Let's look at the following example to illustrate:

The book Mechanics of fatigue by Vladimir V. Bolotin is located on the shelf at 620.1126 BOL.

This number, 620.1126 BOL, is the call number (shelf number) which can be found on the spine (back) of the book.

Let's look at the number 620.1126 BOL by breaking it down:

	620		This part of the number indicates the main class number, consisting of three digits. In this case the 620 is the main class for Mechanical Engineering.
	.		The main class number is followed by a decimal point.
	1126		The decimal point is followed by numbers to indicate sub-divisions of the main class.
	BOL		The sub-division number is followed by three alphabetical letters (normally the first three letters of the author's name or the title of the book). In this case BOL = Bolotin (the author's surname).

Make sure you understand the order of books with such numbers:

	301.4120968 PAT		is before		301.4120968 RAM
	620.1126 BOL		is before		620.1127 BOL.
	621.0012 ZWI		is after		620.1127 BOL
	311.986 SOU		is before		460 SMI

Note: Sometimes there are codes before the actual number, e.g. REF, OS, GB, etc. These indicate collections or branches.