Consider a three-layer flow where the lower layer is denser than the upper layer by virtue of being cooler, but is also fresher. The effect of temperature on density (causing the lower layer to be denser) is therefore partially counteracted by the effect of salinity, since fresh water is lighter than salty water. The middle layer is a transition region. The compensating effects that salinity and temperature have on density makes double diffusive instabilities possible. These instabilities, and the resulting turbulence, are a potentially important cause of mixing in the ocean.