Pati-Salam model

The Pati-Salam model is a Grand unified theory proposed in 1974. 

It was proposed by Abdus Salam and Jogesh Pati. 

The Pati-Salam model actually says that the number of quark color charges is 4, not the conventional 3 of red, green and blue, but adding violet, or what was originally lilac. The new violet quarks were identified with leptons. 

Both the Pati-Salam model and the Georgi-Glashow model can be embedded in SO(10) unification. The difference between the two models lies in how the SO(10) symmetry is broken.