In general relativity, it is common to use the comma notation for partial derivatives
$$\frac{\partial g_{\mu\nu}}{\partial x_\rho} = g_{\mu\nu_,\rho}$$
Where did this notation first appear? Was it known in general mathematics before Einstein?
I don't know much about mathematics books, but I traced it back to Synge's book on General Relativity(1960). Could it have arisen earlier?