Double diffusion convection occurs when 2 components of different diffusivity contributes to the density of the system. When the component of higher diffusivity stabilizes the system however the  component of lower diffusivity destabilizes the system, rising and falling structures form and  known as double diffusive salt fingers. Typical example of this type of instability is seen in the oceans where , Salt (S) and heat (T) are the two components ,  T diffuses 100 times faster than S and also it stabilizes the system. Contribution of these components in the density of the water is measured in terms of  density stability ratio(R_rho). For this case Salt fingers form when 1