Asus P5N-D - Manual (Page 36)

2-12 Chapter 2: Hardware information
5. Rotate each fastener clockwise to
ensure correct orientation when
reinstalling.
Narrow end of the groove
Refer to the documentation in the boxed or stand-alone CPU fan package for
detailed information on CPU fan installation.
The narrow end of the
groove should point
outward after resetting.
(The photo shows the
groove shaded for
emphasis.)