Now, technically, what they mean when they say negative pressure is a negative pressure difference and what a negative pressure difference is, when we subtract the high pressure and the low pressure, we get a negative value because the change in pressure values is a negative quantity. So you should be careful in using this term negative pressure because negative pressure doesn't make sense. But negative pressure difference does, in fact, make sense. So let's take a look at the following diagram, which describes what I just discussed. So this diagram describes the curve for Boils Law. So the y axis is the pressure inside the intraplural space and the x axis is the volume inside that intraplural space.