A letter written (often by the scribe) to tell the illuminator which initial or littera notabilior to supply. Indications concerning which color was to be used (color notes) and fuller notes relating to the subject matter of an image might also be given, often in hard point or metal point to render them less obtrusive.
Lettera guida; Letterina guida
Letra de aviso
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).