Scribal abbreviations, or Sigla (singular: siglum and sigil) are the abbreviations used by ancient and medieval scribes writing in Latin, and later in Greek and Old Norse. Modern manuscript editing (substantive and mechanical) employs sigla as symbols indicating the location of a source manuscript and to identify the copyist(s) of a work.
Abbreviated writing, via sigla, arose partly from the exigencies of the workable nature of the materials — stone, metal, parchment, et cetera — employed in record-making, and partly from their availability.
More information, and examples of abbreviations, on the Wikipedia page [1]