A student asked me today why, if the orthograph's sound is dependent on vocalic context, does OE have both eth and thorn? The letters are not phonemically different after all.
I was stumped. I know that thorn comes from the runic alphabet. Eth does not seem to come either from the A-S runes or from earlier Phoenician/Greek/Latin alphabets. I know ON has both eth and thorn, but I believe the ON situation is the same as in OE.
I can explain why OE has both K and C, or both yogh and G, but whence eth? Knowing its origin might explain its presence alongside thorn.
I believe eth is based on the Irish /d/, which represented [d] and also [edh], depending on the phonological environment. The slash seems to notate a fricative sound but the usual Irish notation for that is a punctus elevatus, so I'm not sure where the slash came from.
I also seem to remember that eth and thorn are positional variants, somewhat like the two forms of Greek /s/. In the BEOWULF MS, thorn is most common initially in stressed words (where it would always be voiceless), with edh more common medially and finally.
Can't think where I first encountered this information. Maybe an expert can clarify why scribes found it useful to have these two letter forms.
It's often said that early scribes avoided the runes because of their association with paganism and its ceremonies and practices, hence uu for w and a crossed d for the th sounds, voiced and voiceless. The church in Iceland, which was influenced by the English in its MS traditions, got the use of runes made illegal there. Eventually scribes grew more tolerant, or the wider spread of literacy among churchmen brought writing to less hardcore monks.
> Aren't they just voiced and unvoiced???
In modern linguistic usage, yes, but in OE MSS, edh is often used for the voiceless fricative at the end of the word as well as for the voiced fricative in a voiced word-medial environment. In the BEOWULF MS, you sometimes find edh in the usual location for thorn (initially) and you sometimes find thorn in the usual position for edh. Consistent use of thorn and edh for the English voicing distinction is a modern practice.
One of the most useful short accounts I've come across of these letter forms is by Chaplais:
‘Who Introduced Charters into England? The Case For Augustine’ pp. 88-107 [III, 10, October 1969] p.96, fn. 58:
‘Two original Italian charters show that in the mid-sixth century the Ostrogoths of Italy used several runes, including the thorn and wynn, in their vernacular subscriptions . .. The thorn and wynn were two of the runes which, according to Gregory of Tours, Chilperic I introduced into the Merovingian scripts . . . On English coins of the seventh century the thorn and other runes are found, sometimes mixed with Roman letters . . . but in English manuscripts the eth, thorn and wynn and the tironian et do not occur until the second half of the eighth century. In the few original Anglo-Saxon charters which have survived for the last quarter of the seventh century the sounds th and w are represented by the letters d (or th) and uu (or u) even in vernacular names . . . As late as the second quarter of the ninth century the sounds th and w were still represented by th and uu as often as by the appropriate runes . . . In Anglo-Saxon charters the thorn seems to have been introduced later than the eth and wynn’.
In England the rune wynn was first used as an additional letter in an uncial charter of 692. The first dated text in which thorn appears is a charter of 811, but in the Corpus glossary it is already more frequent than eth. There is, however, an important geographical difference. There seems to have been resistance to the use of wynn and thorn in the north, where runic literacy survived longest within a Christian context. In the Lindisfarne Gospels and the Vespasian Psalter eth is common, but thorn appears only rarely, mostly as the abbreviation crossed thorn (that).
There is a similar situation for AS coins: no runes are found on Northumbrian coins before the reign of Eanred (807-841). In Mercia coins with runic inscriptions appeared before the end of 7th century (see R. I. Page, An introduction to English Runes).
Runes as additional letters were avoided in manuscripts from the north almost certainly due to their associations with runic literacy. However, this is not necessarily because of their pagan associations, but probably exactly because runic literacy was a living thing in the north. Pagan uses of runes in England are well documented (e.g. a story told by Bede about a prisoner Imma). But nevertheless runes were embraced by the Christian Church after the middle of the 7th century, particularly in the North (in the 8th century they are largely missing from the south and Midlands). In Scandinavia they also came to be used on Christian monuments. However, in both England and Scandinavia they were used almost exclusively as an epigraphic script, and hardly ever in manuscripts, until the early modern period in Scandinavia. In this way runes were kept apart from the Latin alphabet, with the exception of some special cases such as personal names (e.g. a woman’s name written in runes appears scratched in the margin of an 8th-century AS gospels, National Library of Russia LAT. F.v.I.8 (see my review of its facsimile on CDROM in Medium Aevum). An aggressive resistance to the use of runes by the Christian church in Scandinavia also happens largely in the early modern period.
Should mention in regard to distribution that David Megginson has a really interest section of his dissertation in the distribution of ð and þ.
I'm not a linguist, but if you are considering an Irish origin, wouldn't a British/Welsh origin make more sense? The d or dd is very common in Old Welsh and represents the -th sound like the kingdom of Gwynedd voiced as Gwynith (like the actress Gwynith Paltrow). Gododdin -> modern Lothian is another example. Considering how many Welsh personal names (like Caedmon) the English picked up such a transfer is not unlikely.
There's a pretty good Wiki article on this -It's nice to see that ANSAXNET remains such a powerful scholarly resource.From Wiki sub Eth http://en.wikipedia.org/wiki/Eth
Eth (Ð, ð; also spelled edh or eð) is a letter used in Old 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 dh and later d. The capital eth resembles a D with a line partially
through the vertical stroke. The lowercase resembles an insular d with a
line through the top.
The letter originated in Irish writing (Freeborn 1992, 24) as a d with a
cross-stroke added. The lowercase version has retained the curved shape of a
medieval scribe's d, which d itself in general has not (but see for instance
the Audi logo).
In Icelandic, ð represents a voiced dental fricative like th in English
"them", but it never appears as the first letter of a word. The name of the
letter is pronounced eθ, i.e., voiceless, unless followed by a vowel. It has
also been labeled an "interdental fricative."
In Faroese, ð isn't assigned to any particular phoneme and appears mostly
for etymological reasons; however, it does show where most of the Faroese
glides are, and when the ð is before r it is in a few words pronounced as
[g]. In the Icelandic and Faroese alphabets, ð follows d.
In Olav Jakobsen Høyem's version of Nynorsk based on Trøndersk, the ð is
always silent and is introduced for etymological reasons.
In the orthography for Elfdalian, the ð represents a voiced dental fricative
like th in English "them", and it follows d in the alphabet.
In Old English, ð (referred to as ðæt by the Anglo-Saxons) was used
interchangeably with þ (thorn) [*] to represent either voiced or voiceless
dental fricatives. The letter ð was used throughout the Anglo-Saxon era, but
gradually fell out of use in Middle English, disappearing altogether by
about 1300; þ survived longer, ultimately being replaced by
the modern digraph th by about 1500.
The ð is also used by some in written Welsh to represent the letter 'dd'
(the voiced dental fricative).
 American Heritage Dictionary
How to make the Eth
Förslag till en enhetlig stavning för älvdalska (March, 2005) (Swedish)