Eth (ɛð, uppercase: Ð, lowercase: ð; also spelled edh or eð), known as ðæt in Old English, is a letter used in Old English, Middle English, Icelandic, Faroese (in which it is called edd), and Elfdalian.
It was also used in Scandinavia during the Middle Ages, but was subsequently replaced with , and later .
It is often transliterated as .
The lowercase version has been adopted to represent a voiced dental fricative in the International Phonetic Alphabet.
In Old English, (called ðæt) was used interchangeably with to represent the Old English dental fricative phoneme θ or its allophone ð, which exist in modern English phonology as the voiced and voiceless dental fricatives both now spelled .
Unlike the runic letter , is a modified Roman letter. Neither nor was found in the earliest records of Old English. A study of Mercian royal diplomas found that (along with ) began to emerge in the early 8th century, with becoming strongly preferred by the 780s.