Theta-Delta is the most widely used symbol in representing Therianthropy. The symbol consists of two Greek letters layered on top of each other: the Delta and the Theta characters. Both characters are typically depicted in the same color against a contrasting single color background, and may be styled further by intertwining the letters.
History as a Therian Symbol
Theta Delta was invented in 2003 in an early version of The Werelist forums. In one of the threads the topic of creating a representational symbol was raised. The intent was to make a universal Therian symbol for the entire community, and after considering various ideas, the Theta Delta was selected. The symbol was seen as appropriate due to a number of reasons, including logo simplicity and appropriate symbolism. Theta, being a T, represented the word "Therian" and Delta represented change, as in a shift. (citation needed).
Afterwards a Werelist member made a web page detailing the Theta Delta logo, distributing image files to be customized and used freely. Unfortunately the original forum thread disappeared in a server crash a few months later and nearly all the web pages explaining the theta-delta symbol were lost, having not been archived.
Many Therians adapt the basic design into a unique charm with personal significance to them. One of its original uses was to be optionally worn, as an identifying symbol, so that other Therians would recognize the wearer as another Therian, and not feel awkward about starting a conversation about Therianthropy with the wearer.
Historical Symbolism & Meanings
Delta (uppercase Δ, lowercase δ; Greek: Δέλτα [ðelta: Dhelta]) is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4. Delta was derived from the Phoenician letter Dalet, and in the Ancient Greek language it represented a voiced dental plosive /d̪/. The upper-case letter Δ often denotes difference and can symbolize a change.
Theta (uppercase Θ, lowercase θ or ϑ; Greek: Θήτα [Theta]) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth. In has a value of 9 in the Greek numeric system.
"Theta-Delta Symbol" Werelist Archive, 2008. http://www.werelist.net/archive/index.php/t-15250.html