The observed positions of classical cepheids, RR Lyrae stars, δ Scuti stars and dwarf cepheids in the logg-logTe plane form a continuous sequence, thereby defining the location of maximum instability. The amplitude ratio (the ratio of radial velocity amplitude to light amplitude) is small for variables at the upper end of the instability strip and increases almost linearly towards the lower end of the strip. The theory of radial pulsation predicts the trend of this correlation.