  The quest of the knowledge of the structure and dynamics of the solar
interior has been possible thanks to the study of the resonant acoustic (p)
modes that are trapped in the solar interior.
  Since the solar rotation lifts the azimuthal degeneracy of the resonant
modes, their eigenfrequencies are split into their m-components; where l is the
angular degree, n the radial order, and, m the azimuthal order. This separation
--usually called rotational splitting (or just splitting)-- depends on the
rotation rate in the region sampled by the mode. In the same way, the precise
frequency of a mode depends on the physical properties of the cavity where the
mode propagates. Using inversion techniques the rotation rate, the sound speed
or the density profile at different locations inside the Sun can be inferred
from a suitable lineal combination of the measured modes. But, during the last
year, a particular effort has been done in the extraction of physical
information directly from the combination of frequencies: the large and the
small frequency separations.
