  Observations of the pulsations of stars can be used to infer their interior
structure and test theoretical models. The main sequence $\gamma$ Doradus (Dor)
and $\delta$ Scuti (Sct) stars with masses 1.2-2.5 $M_{\sun}$ are particularly
useful for these studies. The $\gamma$ Dor stars pulsate in high-order $g$
modes with periods of order 1 day, driven by convective blocking at the base of
their envelope convection zone. The $\delta$ Sct stars pulsate in low-order $g$
and $p$ modes with periods of order 2 hours, driven by the $\kappa$ mechanism
operating in the Heii ionization zone. Theory predicts an overlap region in the
Hertzsprung-Russell diagram between instability regions, where 'hybrid' stars
pulsating in both types of modes should exist. The two types of modes with
properties governed by different portions of the stellar interior provide
complementary model constraints. Among the known $\gamma$ Dor and $\delta$ Sct
stars, only four have been confirmed as hybrids. Now, analysis of combined
Quarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows that
the frequency spectra are so rich that there are practically no pure $\delta$
Sct or $\gamma$ Dor pulsators, i.e. essentially all of the stars show
frequencies in both the $\delta$ Sct and $\gamma$ Dor frequency range. A new
observational classification scheme is proposed that takes into account the
amplitude as well as the frequency, and is applied to categorize 234 stars as
$\delta$ Sct, $\gamma$ Dor, $\delta$ Sct/$\gamma$ Dor or $\gamma$ Dor/$\delta$
Sct hybrids.
