Question about the derivation of the charecteristics of the Hamilton Jacobi Equation.

Question

I am reading Evan's book on PDE where he mentions the Hamilton Jacobi Equation as follows: $$G(Du,u_t,u,x,t) = u_t + H(Du,x) = 0$$ where $Du=D_xu = (u_{x_1},u_{x_2},...,u_{x_n}).$ Then writing $q=(p,p_{n+1}),$ $y = (x,t)$ we have $$G(q,z,y) = p_{n+1}+H(p,x)$$ and so $$D_{q}G = (D_pH(p,x),1),D_yG=(D_xH(p,x),0),D_zG=0.$$ This last equation I do not understand fully. First, I think that $$D_qG = D_{(p,p_{n+1})}G=(G_p,G_{p_{n+1}})=(D_{p}H(p,x),1).$$ I don't know what the comma followed by $D_yG$ means. Perhaps someone can explain this?