Nine are mutes: "b, g, d, k, p, t, th, ph, kh". They are called mute, because, more than the others, they sound bad, just as we call a performer of tragedy who sounds bad "voiceless". Three of these are thin ("k, p, t"), three are thick ("th, ph, kh"), and three of them are middle ("b, g, d"). They are called middle, because they are thicker than the thin [mutes, but thinner than the thick And "b" is [the mute between "p" and "ph", "g" between "k" and "kh", and "d" between "th" and "t".