# Principles and Practice of Chromatography - Factors Controlling Retention > Page 13

The *peak
height *(h) is the distance between the *peak maximum* and the *base
line *geometrically* *produced beneath the peak.

The *peak
width* (w) is the distance between each side of a peak measure at 0.6065 of
the peak height (*ca* 0.607h). The peak width measured at this height is
equivalent to two standard deviations (2s) of the Gaussian curve and thus has
significance when dealing with chromatography theory.

The *peak
width at half height * (w_{0.5}) is the distance between each side
of a peak measured at half the peak height.
The peak width measured at half height has no
significance with respect to chromatography theory.

The *peak
width at the base *(w_{B}) is the distance between the intersections
of the tangents drawn to the sides of the peak and the *peak base*
geometrically produced. The peak width at the base is equivalent to four
standard deviations (4s) of the Gaussian curve and thus also has significance
when dealing with chromatography theory.

# Factors Controlling Retention

The equation
for the retention volume (V_{r}), as derived from the Plate theory (see
ThePlate Theory and Extensions
) is as follows,

**V _{r = }V_{m }+ KV_{S}**

_{ }