Graphium: Ancient styli were often of iron or bronze, but are found more often of bone. Although some had a rectangular or oval flat surface at the top, many had simply rounded knobs for smoothing over the wax already scratched with characters. hence the phrase vertere stilum means 'to erase, make a correction.' Cf. Hor. S. I.10.73: Saepe stilum vertas, iterum quae digna legi sint scripturus; Cic. Verr. II.2.101: vertit stilum in tabulis suis; also Prudent. Hymn. S. Cassiani Martyr., 51‑54:

2 utrimque: "either way", i.e. from either end. For a similar use cf. Hor. Epist. I.18.9: Virtus est medium vitiorum et utrimque reductum.

3 revocat: one advantage the pen has over the tongue; witness Horace's famous lament, Epist. I.18.71: semel emissum volat irrevocabile verbum, an idea which he repeated in Ars Poet. 390: nescit vox missa reverti.