The maximum number of phases in a two component system will be four. Maximum number of phases exists when degree of freedom is zero

$P = C – F + 2 \\ = 2 – 0 + 2 \\ = 4$

Maximum degree of freedom in a 2 component system will be 3 when system exists as one phase

$F = C – P + 2 \\ = 2 – 1 + 2 \\ = 3$

System will have three variables namely temperature, pressure and concentration. For constructing a phase diagram of 2-component system, a three dimensional space model is required using these 3 variables as its coordinates.

It is possible to split such a 3D diagram into 2D by keeping the third variable in each case as constant.

It is convenient to prepare temperature – composition (T-C) diagrams keeping pressure constant, such diagrams are called Isobaric.

Similarly, P-C diagrams at constant temperature are called isothermal.

P-T diagrams at constant composition are called isoplethal.

Any such restriction in phase rule equation regarding constancy of one of the variables reduces the phase rule equation to following form –

F = C – P + 1

OR

F + P = C + 1

This is known as reduced phase rule equation