From the observing campaigns of a number of helioseismic
telescope networks such as the Global Oscillation Network Group (GONG) and
also from the Solar Heliospheric Observatory satellite (SoHO),
helioseismologists now have data on in excess of 20000 oscillation
frequencies $\nu$ of the modes known collectively as the 5 minute oscillations.
Depending on the order and degree of the modes these data are available
either as individual frequencies of modes with known radial order n,
spherical harmonic degree l, and order m, or as so-called `a-coefficients'
which are obtained by fitting polynomials to ridges of power in the
(l,nu ) diagram. Although there is a one-to-one relationship between
these two types of data, the conversion between them is not straightforward
which complicates comparison of helioseismic inference between the two types
of data. Their relationship with each other and with the solar rotation rate
is shown as well as some inversion results for the differential rotation rate
of the Sun as a function of radius and latitude.