 So a big assumption that we make in soil mechanics is that the solid phase within this three-phase model is incompressible. That means that it doesn't change volume when we subject it to changes in stress. But soils, we know, can be compacted. They consolidate, they settle, so they change volume. So all of that volume change happens within the void space, within the soil. So we need a way to describe the voids within the soil. And there's three parameters that are typically used. So those three parameters are the void ratio, and that's given the symbol E. The specific volume, and that's given the symbol small v, and porosity. That's given the symbol n. So these three parameters describe the voids within a soil, and they describe it in slightly different ways. So the void ratio in terms of this three-phase model is the volume of the void over the volume of the solids. The specific volume is the total volume over the volume of the solids. And the porosity is the volume of the voids over the total volume. So all of these, as you can see, it's a volume over a volume in all three cases. So all of these are unitless. So don't make the common mistake that you attribute some sort of units to them. Often students put volume units, meters cubed or something, but they're all unitless. And that's important to remember. So another common misconception is what the specific volume is. It's not a volume in and of itself, it's a ratio. And it can be conceptualized as how much volume the soil skeleton has really taken up. So the soil skeleton is a phrase we use to describe the soil grains and how they sort of interact and take force. This specific volume really describes sort of how much volume that takes up. The porosity describes the amount of the total volume that's voids, and that's probably something you've come across before. The void ratio is probably something new to you, and that's the ratio of the volume of the voids over the volume of solids. They all relate to each other through two equations. The relationship between the void ratio and the specific volume is this. So the void ratio is equal to the specific volume minus one, and the porosity is equal to the void ratio divided by the specific volume. That really means that if we have one of these parameters, we can figure out what the other two are.