 So, the pattern to these decay or reaction processes can be summarized like this. For every lepton that's produced, an associated anti-neutrino is produced. For every anti-lepton that's produced, an associated neutrino is produced. For every lepton that decays, an associated neutrino is produced. And for every anti-lepton that decays, an associated anti-neutrino is produced. So, when I say associated neutrino or anti-neutrino, I mean the neutrino or anti-neutrino that belongs to the same family as that charged lepton or anti-lepton. Okay, so what does this pattern potentially mean? Well, let's take muon decay, for example, and make up a quantum number associated with each lepton family. For each family, the lepton in that family gets a plus one, and the anti-lepton gets a minus one. So if you do that for muon decay, what you'll see is on the left side, you have a plus one for the muon family. On the right side, you also have a plus one for the muon family, you also have a plus one for the electron, and a minus one for the anti-electron neutrino. So in this reaction, the lepton number for each lepton family is conserved. As it turns out, all the reactions that involve leptons follow this rule, so this leads to our first new conservation law. In any reaction, the lepton number for each lepton family must be conserved. This conservation law is an addition to the conservation laws we've already learned, either in this module or in previous ones. So that's conservation of charge, conservation of linear and angular momentum, and conservation of energy. In physics, a conservation law is something we've found to be true over and over and over again in many different experiments. If we were to find a reaction that seemed to violate one of these conservation laws, we would of course re-examine it.