And what that basically means is when our substrate concentration is equal to Km, exactly half of all the active sites in the enzyme mixture are filled with the substrate. And this can be seen in the following diagram. So initially we begin with, let's say, an enzyme mixture that consists of four identical enzymes. And these are shown in red. And notice that each one of these enzymes contains this Crevice and that's the active site to which a substrate can actually bind. Now, if we take this enzyme mixture and we add a concentration of substrate that is equal to Km, the Macalus constant, exactly half of these active sites will be filled.