The sheets that have been bound togeter in some collections / books / booklets are numbered rather than the pages i.e. one number applies to both the front and back of the sheet. It is important to realise this when following up a reference, which may not make it clear whether it is to a page or folio number – if the subject of the reference can not be found turn the sheet over.

The number is generally on the front, and when bound of course this sheet appears on the right. Sometimes references specify the side. The front side is referred to as recto, abbreviated simply to r in references, the reverse as verso, abbreviated to v.