That is our reference point. Now, from organic chemistry we know the carbon next to a carbon nil is known as the alpha carbon. So this is the alpha carbon, this is the beta carbon and the carbon all the way at the end on the opposing side of the carboxylic acid. The last carbon is known as the omega carbon given by this Greek symbol omega. So we have alpha beta and then we have omega. We'll see why that's important in just a moment.