In Zee's quantum theory text, introducing the Dirac equation, he states the gamma matrices as direct products of Pauli matrices. The statements involve the identity matrix, sigma matrices, and tau matrices. It took me a bit to realize that the latter were identical. I hadn't seen the tau notation before; it's only sigma in my hardest quantum mechanics text, Messiah. I see that Wikipedia says that tau is used in context of isospin (not mentioned in this chapter in Zee). Can I ask more about the convention, so that I understand why both notations are used in the same equation? They appear a few pages later in straightforward further manipulation as well. That is, I understand the math but not the notation. Thanks!