A sheet of writing material, from the Latin for a leaf, one half of a bifolium. The front and back of a folio are referred to as the recto and verso, respectively. The numbering of leaves, as opposed to pages, is termed foliation and is commonly found in manuscripts. 'Folio' and 'folios' (or 'folia') are often abbreviated as f. and ff., or fol. and fols., The term can also be used to denote a large volume size, in which each sheet is folded only once, and in this context can be abbreviated to f° or 2º; cf. quarto, octavo, etc.