Etymology: in words of Old English or Old Norse origin, and in words from Greek, is a consonantal digraph representing a simple sound, or rather (in Germanic words), a pair of simple sounds, breath and voice, indicated in this dictionary by the letters /θ/ and /ð/; the former, as in thin, bath /θɪn//bɑːθ/, being the breath dental spirant akin to t, and the latter, as in then, bathe /ðɛn//beɪð/, the voiced dental spirant akin to d. The group /t//d//θ//ð/, corresponds to the group /p//b//f//v/. The breath spirant is identical with modern Greek theta (Θ, θ), and approximately with Spanish z (or c before e, i). The Greek letter, which corresponds etymologically to Sanskrit ध dh (and so, by Grimm's Law, to Germanic and English D), was in early inscriptions represented by TH, and was a true aspirate; it was subsequently often written ΤΘ, τθ, and has probably the sound /tθ/; but by the second century b.c. it had sunk into a simple sound, = our /θ/. The Romans, having neither the sound nor the symbol, represented the letter by TH, as in Θάψος, Thapsus, but apparently this was pronounced, at least in late Latin (whence in all the Romanic languages), as simple t; cf. Greek θεωρία, Latin theōria, Italian and Spanish teoria; in Portuguese theoria, French théorie, spelt with th, pronounced with t; also Greek Θωμᾶς, Latin Thōmās, Italian Toma, Spanish Tomás; Portuguese, French, English Thomas all pronounced with T. (2) In Germanic the breath spirant /θ/ was very frequent, being the regular etymological representative of Indo-European t initially or after the stressed vowel, as in Old Germanic *þrijiz, Gothic þreis, Old English þreo, English three, = Indo-European *treies, Sanskrit trayas, Greek τρεῖς, Latin trēs; Old Germanic *brôþer, Gothic brôþar, Old English brōþōr, brōðor, English brother, = Indo-European ˈbhrātēr, Greek ϕράτηρ clansman, Latin ˈfrāter. The voiced spirant in brōðor, etc., was a later development (c700 in English) from the breath sound between vowels or voiced consonants, as in the parallel v and z from f and s. Initially, the same change of /θ/ to /ð/ took place during the Middle English period in the demonstrative group of words, the, that, and their kindred, this, these, †tho, those, there, then, than, thence, thither, thus, etc., and in the pronouns of the second person singular, thou, thee, thine, thy: these constitute the only words in English with initial /ð/. In the same group of words in the cognate Germanic languages /θ/ has passed through /ð/ into /d/; thus German das, Dutch dat, Danish, Swedish det ‘that’; in High German, Low German, and Dutch the same has taken place even in other original th words which retain /θ/ in English; e.g. German dach, denken, ding, dick, donner, drei = English thatch, think, thing, thick, thunder, three.(3) In the demonstrative and pronominal groups of words, change of initial þ to t, by assimilation to a preceding dental (t, d, s), appears in earlier English. Old English þæt þe became þæt-te, þætte; þe læs þe appears in the 11th cent. as þe læste, whence modern lest. In the last section of the Old English Chronicle, from 1132, þe after t or d regularly becomes te (e.g. þat te king, and te eorles). In the Ormulum and the Cotton MS. of Cursor Mundi, this assimilation is seen in all the words of the the-thou group ( Orm. þatt tatt te goddspell meneþþ, wrohht tiss boc, and tatt te follc all þess te bett; Cursor, ne was tar, here and tare, scho serued taim, als sais te sau). So in Ancren Riwle (and tet is, et tesse uerse, þeo þet tus doð, and tes oðer, etc.). In the course of the 14th cent., this assimilation was given up, and the spirant reappeared (as ð). (4) In the Runic alphabet (futhorc) the breath spirant had to itself a symbol Þ or þ (called thorn); but in the earliest known Old English writings in the Roman alphabet this was represented by th, the voiced spirant being often represented by d (∂) (sometimes by th). Before 700 probably, the character ð, formed by a bar across the stem of ∂, was introduced; it appears in a charter of Wihtræd, king of Kent, 700–715 (Sweet Oldest English Texts 428). Apparently it was first used to denote the voiced spirant: see the proper names in the Moore manuscript of Bæda, c737, and the Liber Vitæ, Cott. MS., c800, and charters before 800 generally. But in the ninth century it was used for both spirants, as in the Vespasian Psalter, c825 (e.g. iv. 5 ða ðe cweoðað), and in a West Saxon charter of 847 ( O.E.T. 433). In the 8th century apparently, the thorn, þ, was adopted from the Runic futhorc, the earliest charter showing it being one of Coenwulf, king of Mercia, of 811 ( O.E.T. 456); but it was not much used till late in the 9th cent. A Surrey charter a889 ( O.E.T. 451) has 34 examples of ð initial, and 25 medial or final, with 49 of þ initial, and 1 medial. From the later years of the 9th cent. ð and þ were used promiscuously in West Saxon works, with some preponderance of þ initially and ð finally. This continued in Middle English till the 13th cent. On the other hand, the Durham Rituale and the Lindisfarne Gospel Gloss, c950, have uniformly ð in all positions (except in the compendium  for ðæt), as has also the East Anglian Genesis & Exodus, c1250; while the Mercian portion of the Rushworth Gospel Gloss, c975, and Ormin, c1200, have only þ. After 1250 the ð speedily became obsolete; þ remained in use, but was gradually restricted more or less to the pronominal and demonstrative words. In later times its manuscript form approached, and at times became identical with, that of y (the latter being sometimes distinguished by having a dot placed over it). As the continental type used by Caxton had no þ, its place in print was usually supplied by th for both sounds and in all positions. But in Scotland, the early printers, especially in the demonstrative and pronominal words, continued the þ as y, as in y^e, yis, yat, you (= thou), a practice also common in England in manuscript, and hardly yet extinct. Confusion with the modern y consonant, Middle English ȝ, was avoided in Scotland, sometimes by writing the latter yh, but usually by continuing Middle English ȝ in the form g or z, so that ye zeir stood for þe ȝeir, i.e. the year. It is remarkable that, when Old English þ and ð were both in use, no attempt was made to differentiate them as breath and voice spirants, and apparently no serious attempt even to distinguish them as initial and medio-final, as was done in Norwegian when the Roman alphabet was adopted, c1200, and in Icelandic before 1300. At an earlier date (probably c800) the character ð was partially adopted from Old English in Old Saxon, and was used generally in the middle and end of words, while th was usual as the breath spirant initially.(5) In a few compounds, as anthill , outhouse , lighthouse , Chatham , Wytham , Yetholm , etc., t and h come together but do not form a digraph; and in a few foreign words, chiefly East Indian, as thakur , thug , th represents Sanskrit थ th or ठ ṭh , the sound being a t or ṭ followed by a slight aspiration (t^h, ṭ^h), in English commonly reduced to t . In a few proper names and other words derived from or influenced by French, as Thomas , Thompson , thyme , th is pronounced as t ; several other words were formerly so treated, and even spelt with t , e.g. theatre , theme , theology , throne , authentic , orthography : t has become fixed in treacle , treasure . The late Latin and Romanic treatment of th as t often led to the spelling th where t was etymological, as in Thames , Sathan ; in amaranth , amianthus , author , etc., the corruption has also affected the pronunciation. See the individual words. In some Middle English manuscripts th frequently appears for t or for d : e.g. tho to, thyll till, myghth might, nyghth night, whythe white; thede deed, theer deer, thegree degree, thepartyth departed, tho do, thogh doth, abothe abode, groundeth grounded, iclodeth y-clothed, lowthe loud, rothe rood, unther under. Early Middle English scribes (probably Norman) often confounded the English letters þ (or ð) and ȝ, writing e.g. ȝefinge for þefinge , thieving, wiȝ , worȝ , wroȝ for wiþ , worþ , wroþ (in Auchinleck MS. of Florice & Bl.). (6) Etymologically, modern English th (ð) often represents an Old English d , esp. before r or er , as in father , mother , gather , hither , together , etc.; dialectally, this sometimes extends to other words, as bladder , ladder , solder ; on the other hand some dialects retain original d , and extend it to other words, as brother , further , rather , southern-wood , wether . In burden and murder , d represents the earlier ð of burthen , murther . Dialectally th is sometimes substituted for f , and vice versa: e.g. thane , thetch , thistolow , thrail , thrae , throm , thurrow , for fane , fetch (vetch), fistula , frail (flail), frae , from , furrow ; also fill , Fuirsday , for thill n.¹, Thursday n. and adv. The Welsh name Llewelyn appears in English as Thlewelyn ( Rolls of Parl. I. 463/1, Edw. I or II), and Fluellen (Shakespeare Hen. V). Th also occurs dialectally for wh, as in thirl, thortleberry, thorl, for whirl, whortleberry, whorl. Conversely, Sc. has whaing, whang, white, whittle, for thwaing, thwang, thwite, thwittle.

Etymology: Symbolic abbreviation for thorium n.