A sicilicus was an old Latin diacritical mark, , like a reversed C (Ɔ)Cf. John Edwin Sandys, A Companion to Latin Studies, Cambridge University Press 1910, §1099, p. 743, where specific instances are provided: , , . placed above a letter and evidently deriving its name from its shape like a little sickle (which is sicilis in Latin). The ancient sources sayCf. Isidore Etymologiae 1.27.29 (); Nisus fr. 5 Mazzarino in Velius Longus de Orthographia Keil 7.80; Gaius Marius Victorinus Ars Grammatica 4.2 Mariotti. that during the time of the Republic it was placed above a geminate consonant to indicate that the consonant counted twice, although there is hardly any epigraphic or paleographic evidence available from such an early time. When such geminate consonants began to be represented during classical times by writing the letter twice, the sicilicus naturally fell into disuse in this function, but continued to be used to indicate the doubling of vowels as an indication of length, in the developed form of the apex.Apex and Sicilicus, Revilo P. Oliver, The American Journal of Philology, Vol. 87, No. 2. (Apr., 1966), pp. 156-58. For a counter-view see Michael Fontaine, Sicilicissitat (Plautus, Menaechmi 12) and Early Geminate Writing in Latin (with an Appendix on Men. 13). Mnemosyne, Volume 59, Number 1 (2006) pp. 104-5. Fontaine suggests that Plautus alludes to the sicilicus in the prologue to Menaechmi.Michael Fontaine, Sicilicissitat (Plautus, Menaechmi 12) and Early Geminate Writing in Latin (with an Appendix on Men. 13). Mnemosyne, Volume 59, Number 1 (2006) pp. 95-110.
See also Open O, although this is a full letter, and not a diacritic placed above a letter Antisigma, although this is a full letter, and not a diacritic placed above a letter Apex (diacritic), used for long vowels instead of long consonants Apostrophe, whose shape is derived from it Comma (punctuation), whose shape is similar Latin spelling and pronunciation
References Lewis and Short Latin Lexicon
Notes