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bifolium

bifolium Palæogr.
  (baɪˈfəʊlɪəm)
  Pl. -ia.
  [f. bi-2 + L. folium leaf.]
  A pair of conjoint leaves.

1938 E. A. Lowe Codices Latini Antiquiores iii. 283 Foll. 3..composed of a bifolium and a single leaf. Ibid. 284 Ruling on the hair-side central opening, four bifolia at a time before folding. 1957 N. R. Ker Catal. MSS. containing Anglo-Saxon 407 A bifolium followed by three singletons. 1960 E. A. Lowe English Uncial 11 The following Insular, at least non-Italian, features were enumerated: pricking and ruling of the bifolia after folding, [etc.].

Oxford English Dictionary

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