  Spinning neutron stars can emit long-lived gravitational waves. There are
several mechanisms that can produce such continuous wave emission. These
mechanisms relate to the strains in the elastic crust, the star's magnetic
field, superfluidity of the neutron fluid, and bulk oscillations of the entire
star. In this chapter we describe how the frequency content of the
gravitational wave signal, and its relation to any electromagnetically observed
spin frequency, can be used to constrain the mechanism producing the
gravitational waves. These ideas will be of use in the event of the first
detections of such signals, and help convert a detection into useful physical
insight.
