The covariant derivative in standard model is given by Dμ = ∂μ + igs Gaμ La + ig Wbμ Tb + ig'BμY where Gaμ are the eight gluon fields, Wbμ the three weak interaction bosons and Bμ the single hypercharge boson. The La's are SU(3)C generators (the 3×3 Gell-Mann matrices ½ λa for triplets, 0 for singlets), the Tb's are the SU(2)L generators (the 2×2 Pauli matrices ½ τb for doublets, 0 for singlets), and the Y's are the U(1)Y charges. It looks like the second term is a 3×3 matrix, the third term is a 2×2 matrix, and the last term is a single term. How can they add up? Is Dμ a 3×3 matrix or something else?