# Partial Differentiation [Subscript]

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
Staff Emeritus
It means "holding z constant".

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