component does have a different centroid.

We now turn to the difficulty of reducing these profiles to a single parameter, namely velocity. There have been two approaches to measure velocities. The first fits a single Gaussian to a line profile and ascribes the centroid of the Gaussian to the velocity of the feature. This method is used by many studies, including . The alternative is to measure the wavelength of maximum absorption (usually after some smoothing) to represent the velocity of the feature. This is the method described by @Blondin06 and used by @Foley11:vgrad and @Foley12:vel. Although there are many arguments to use either method, we will focus on the potential systematic errors of using these methods when a feature has multiple components like the Ca H&K feature.

In Figure \[f:dgauss\], we also show a single-Gaussian fit to the Ca H&K feature. Besides being a poor representation of the data, the centroid of the Gaussian is consistently intermediate to the two components. Usually, one wants to measure the photospheric velocity for a given feature. With that goal, the single Gaussian clearly fails. A single Gaussian, by measuring something intermediate to the two components, measures nothing physical. Furthermore, the centroid of 