In general, the phase rule equation is

$P + F = C + E$

where E is the number of external variables such as temperature, pressure and in unusual cases, electric field and magnetic field.

A solid -liquid equilibrium of an alloy has practically no gas phase, and the effect of pressure is negligible on this type of equilibrium.

Thus, keeping the pressure constant of a system in which vapour phase is not considered is known as a condensed system.

As such, the phase rule becomes, $F = C - P + 1$

This is known as the condensed rule.