In a pressure-volume diagram ($$\mathit{\Pi}-V$$ diagram), an adiabatic curve $$\sim V^{-\gamma}$$ was plotted. The product $$\mathit{\Pi}_1 \, {V_1}^{\gamma}$$ BEFORE the adiabatic process is equal to the product $$\mathit{\Pi}_2 \, {V_2}^{\gamma}$$ AFTER the adiabatic process.