Partial Differentiation [Subscript]

[Air]

What does the subscript outside a partial differentiation mean? For example:

\left(\frac{\partial x}{\partial y}\right)_z

What does the z mean above?

[cristo]

It means "holding z constant".

[Amos Axiom]

Is that any different from a regular partial derivative though? I've started to encounter the notation in literature and am finding it strange.

[Ray Vickson]

Is that any different from a regular partial derivative though? I've started to encounter the notation in literature and am finding it strange.

It is rarely used in Mathematics, but is used often in some fields such as Thermodynamics. Basically, it indicates that we are considering x to be a function of y and z, without using notation like x(y,z).

RGV