Tau gallicum

Tau gallicum, or D with short stroke overlay in Unicode, (majuscule: Ꟈ, minuscule: ꟈ) is a letter that was used to write the Gaulish language. It is a D with the horizontal bar from the Greek letter Θ. It likely represented a or  sound, like the ts in cats or the st in stop. In modern times, it is used in the Cubeo and Daasanach languages to represent the sound, in the Northern Embera language to represent the sound , and in the Moro language.

Name
The Latin phrase tau gallicum literally means "Gallic tau". The only known mention of the letter is found in Catalepton, a set of epigrams attributed to Virgil and collected after his death in Appendix Vergiliana. The second epigram contains the following text: Corinthiorum amator iste uerborum, iste iste rhetor, namque quatenus totus Thucydides, tyrannus Atticae febris: tau Gallicum, min et sphin ut male illisit, ita omnia ista uerba miscuit fratri.

— Virgil Catalepton II

It is not known, however, whether the sound described by Virgil is the same as that for which the term is currently used.

Letter
After using the Greek alphabet, the Gauls adopted the Latin alphabet to transcribe their language. However, they keep a few letters from the previous alphabet to note sounds unknown to the second. Tau gallicum is said to have been inspired by the Greek letter Θ (theta). Its spelling is varies between $⟨Ꟈ⟩$ and $⟨ꟉꟉ⟩$.

The letter can be found in the initial of the name of the Celtic goddess Sirona, whose name is written as: Sirona, Ꟈirona or Thirona, highlighting the difficulty of noting the initial sound in the Latin alphabet.

The letter is also present in the lead of Chamalières, a lead tablet discovered in 1971 in Chamalières and written in the Gallic language with Latin cursive letters: snIeꟈꟈdic, aꟈꟈedillI.

Pronunciation
The precise value of the sound transcribed by the Gallic tau is not known. It is supposed that it denotes an alveolar affricate, in free variation with in initial position.

Use on computers
The letter is encoded into Unicode as and  respectively.