This slowly developed from the fifth-century form resembling the Greek letter tau in the hands of medieval Irish and English writers.
ðɪs slˈoʊli dɪvˈɛləpt fɹʌmðə fˈɪfθsˈɛntʃɚɹi fˈɔːɹm ɹᵻzˈɛmblɪŋ ðə ɡɹˈiːk lˈɛɾɚ tˈaʊ ɪnðə hˈændz ʌv mˈɛdɪˌiːvəl ˈaɪɹɪʃ ænd ˈɪŋɡlɪʃ ɹˈaɪɾɚz.