Of the letters that are called “voiceless”, of which there are nine, three are smooth, three rough and three intermediate. The smooth letters are κ, π, and τ, the roughθ, φ, and χ; the intermediate β, γ, and δ. They are severally pronounced as follows: three of them, π, φ and β, from the edge of the lips, when the mouth is pressed shut and the breath is projected from the windpipe and breaks through the barrier. Of these letters π is smooth, φrough and β intermediate between the two, being smoother than the latter and rougher than the former. This is one group of voiceless letters, all three letters being pronounced with a similar configuration ⟨of the mouth⟩, but differing in smoothness and roughness. ― translation from: Stephen Usher, opere citatoLCL 466 (1985), chapter xiv, paragraph 6, page 103, lines 3–16