Mob spawn areas are nonintuitive. This article aims to help describe them.

Width and height work basically the same, so only one will be described here. What applies to west, applies to north. What applies to east, applies to south. Here is an overview of how it works:

	West					East
Tiles	...	4	3	2	1	0	1	2	3	4	...
Width	...	8	6	4	2	0	1	3	5	7	...

So, a width of zero restricts the area to the column of tiles specified by it's location, while a width of three extends it one tile west, and two tiles east. A width less than zero will prevent the map server from loading.

If both the x- and y-coordinates of the area are zero, the width and height are ignored and all individuals can spawn anywhere on the map. If only one is zero, the mob won't be used.