A Koenig Born diagram is a diagram used to remember the

Maxwell's relations.
In the

*figure* below , U stands for

Internal Energy, H for

Enthalpy, G for

Gibb's free energy, and A for the

Helmholtz free energy, V is

Volume, P is

pressure, T is

temperature and S is

entropy.
These are arranged in a Koenig Born diagram as follows:

V A T
U G
S H P
In addition there is an arrow from S to T and from P to V.

The diagram is read as follows. For example the top row of the square would be read as dA is a function of dV and dT and dA=-PdV-SdT.

The coefficient of dV is -P because the arrow links V to P and the connection goes against the arrow.Similarly the coefficent of dT is S because the arrow links T to S and the the minus sign is because the connection goes against the arrow.