What the above equations express is that given an initial pressure P1 and volume V1 at time 1 the product of these two values will be equal to the product of the pressure P2 and volume V2 at a later time 2. This means that if there is an increase in the volume of cavity, there will be a corresponding decrease in pressure of that same cavity, and vice versa. In other words, volume and pressure are inversely proportional (or negatively correlated) to each other.