At one point, early on, Taw/Daw was pronounced /t/ and Delta was pronounced as /d/. In a nutshell, due to a bunch of sound shifts that occurred in the language, Taw (which is classified as a “voiceless stopف) started being voiced. A voiced /t/ is actually a /d/ - that’s the only difference between the two sounds. The letter “thetaف came to ‘replace’ Taw as far as pronunciation was concerned; theta was at one time /t/ said with a puff of air with it (like the ‘t’ in English “teaف). The accompanying puff of air (called aspiration) was dropped and the letter was pronounced as /t/ about the same time as Taw became pronounced as /d/. Delta already existed in the alphabet and was simply kept (it’s used mainly in Greek loans anyway). English has “duplicateف sounds for letters as well – when you think about it, we really don’t need the letter ‘c’; it can be replaced with ‘k’ or ‘s’, but for historical reasons, it’s kept. Same with delta in Coptic.
this answer by Szelog, Mike and it was originally posted in http://groups.yahoo.com/group/RemEnKimi/message/5940
