Old Turkic
Old Turkic (more exactly East Old Turkic, in order to distinguish from West Old Turkic) is the earliest attested form of the Common Turkic languages, first found in Second Turkic Khaganate then in Uyghur Khaganate inscriptions. In marked contrast to Middle Turkic, the geographic extent of (East) Old Turkic is rather confined, being limited mainly to East Turkistan (Old Uyghur) and Mongolia (Orkhon Turkic). In terms of the datability of extant written sources, the period of Old Turkic can be dated from slightly before 720 AD to the Mongol invasions of the 13th century. Orkhon Turkic and Old Uyghur are considered to be dialects of East Old Turkic, Orkhon Turkic being the earliest attested dialect of (East) Old Turkic. There is a difference of opinion among linguists with regard to Karakhanid Turkic (spoken in Kara-Khanid Khanate), some (among whom are Omeljan Pritsak, Sergey Malov and most importantly Marcel Erdal) classify it as another dialect of East Old Turkic, while others prefer to include Karakhanid among Middle Turkic languages; nonetheless, Karakhanid is extremely close to Old Uyghur so much so that a single grammatical description will fit both of them.
Old Turkic (more exactly East Old Turkic, in order to distinguish from West Old Turkic) is the earliest attested form of the Common Turkic languages, first found in Second Turkic Khaganate then in Uyghur Khaganate inscriptions. In marked contrast to Middle Turkic, the geographic extent of (East) Old Turkic is rather confined, being limited mainly to East Turkistan (Old Uyghur) and Mongolia (Orkhon Turkic). In terms of the datability of extant written sources, the period of Old Turkic can be dated from slightly before 720 AD to the Mongol invasions of the 13th century. Orkhon Turkic and Old Uyghur are considered to be dialects of East Old Turkic, Orkhon Turkic being the earliest attested dialect of (East) Old Turkic. There is a difference of opinion among linguists with regard to Karakhanid Turkic (spoken in Kara-Khanid Khanate), some (among whom are Omeljan Pritsak, Sergey Malov and most importantly Marcel Erdal) classify it as another dialect of East Old Turkic, while others prefer to include Karakhanid among Middle Turkic languages; nonetheless, Karakhanid is extremely close to Old Uyghur so much so that a single grammatical description will fit both of them. East Old Turkic and West Old Turkic together comprise the Old Turkic proper. East Old Turkic is the oldest attested member of the Siberian Turkic branch of Turkic languages, and several of its now-archaic grammatical as well as lexical features are extant in the modern Yellow Uyghur, Lop Nur Uyghur and Khalaj (all of which are endangered); Khalaj, for instance, has (surprisingly) retained a considerable number of archaic Old Turkic words despite forming a language island within Central Iran and being heavily influenced by Persian. Old Uyghur is not a direct ancestor of the Modern Standard Uyghur language (also called New Uyghur); the contemporaneous ancestor of Modern Uyghur was one of the Middle Turkic languages, later giving rise to Chagatai literary language (although Modern Uyghur does retain some features of Old Uyghur whereas Chagatai almost did not influence the spoken vernacular).
Old Turkic is attested in a number of scripts, including the Old Turkic script, the Old Uyghur alphabet, the Brahmi script, and the Manichaean script. The Turkic runiform alphabet of Orkhon Turkic was deciphered by Vilhelm Thomsen in 1893.
Old Turkic often refers not to a single language, but collectively to the closely related and mutually intelligible stages of various Common Turkic languages spoken during the late first millennium.

Sources
In stark contrast to Middle Turkic texts, the vast majority of available Old Turkic texts comes from non-Muslim sources. The sources of Old Turkic are divided into two (three according to Marcel Erdal) corpora:

the 8th to 10th century Orkhon inscriptions in Mongolia and the Yenisey basin (Orkhon Turkic).
9th to 13th century Uyghur manuscripts from Gansu and Xinjiang (Old Uyghur), in various scripts including Brahmi, Tibetan, Syriac and Sogdian alphabets, treating religious (Buddhist, Manichaean and Church of the East), legal, literary, folkloric and astrological material, as well as personal correspondence.
(According to Marcel Erdal) Kutadgu Bilig by Yūsuf Balasaguni and the parts of the monumental Dīwān Lughāt al-Turk by Mahmud al-Kashgari. As Marcel Erdal acknowledged, the latter work is not wholly Old Turkic but also contains lexemes from Middle Turkic, thus providing a rich resource for the study of both Old and Middle Turkic.

Writing systems
The Old Turkic script (also known variously as Göktürk script, Orkhon script, Orkhon-Yenisey script) is the alphabet used by the Göktürks and other early Turkic khanates during the 8th to 10th centuries to record the Old Turkic language.The script is named after the Orkhon Valley in Mongolia where early 8th-century inscriptions were discovered in an 1889 expedition by Nikolai Yadrintsev.This writing system was later used within the Uyghur Khaganate. Additionally, a Yenisei variant is known from 9th-century Yenisei Kirghiz inscriptions, and it has likely cousins in the Talas Valley of Turkestan and the Old Hungarian alphabet of the 10th century. Words were usually written from right to left. Variants of the script were found from Mongolia and Xinjiang in the east to the Balkans in the west. The preserved inscriptions were dated to between the 8th and 10th centuries.

Phonology
Vowel roundness are assimilated thorough the word through vowel harmony. Some vowels were considered to occur only in the initial syllable, but they were later found to be in suffixes. Length is distinctive for all vowels; while most of its daughter languages have lost the distinction, many of these preserve it in the case of /e/ with a height distinction, where the long phoneme developed into a more closed vowel than the short counterpart.

Old Turkic is highly restrictive in which consonants words can begin with: words can begin with /b/, /t/, /tʃ/, /k/, /q/, /s/, /ɫ/ and /j/, but they do not usually begin with /p/, /d/, /g/, /ɢ/, /l/, /ɾ/, /n/, /ɲ/, /ŋ/, /m/, /ʃ/, or /z/. The only exceptions are 𐰤𐰀 (ne, "what, which") and its derivatives, and some early assimilations of word-initial /b/ to /m/ preceding a nasal in a word such as 𐰢𐰤 (men, "I").

Grammar
Cases
There are approximately 12 case morphemes in Old Turkic (treating 3 types of accusatives as one); the table below lists Old Turkic cases following Marcel Erdal’s classification (some phonemes of suffixes written in capital letters denote archiphonemes which sometimes are dropped or changed as per (East) Old Turkic phonotactics):

Grammatical Number
Old Turkic (like Modern Turkic) had 2 grammatical numbers: singular and plural. However, Old Turkic also formed collective nouns (a category related to plurals) by a separate suffix -(A)gU(n) e.g. tayagunuŋuz ‘your colts’. Unlike Modern Turkic, Old Turkic had 3 types of suffixes to denote plural:

Today, all Modern Turkic languages (except for Chuvash) use exclusively the suffix of the -lAr type for plural.

Verb
Finite verb forms in Old Turkic (i.e. verbs to which a tense suffix is added) always conjugate for person and number of the subject by corresponding suffixes save for the 3rd person, in which case person suffix is absent. This grammatical configuration is preserved in the majority of Modern Turkic languages, except for some such as Western Yugur in which verbs no longer agree with the person of the subject.

Tense
Old Turkic had a complex system of tenses, which could be divided into six simple and derived tenses, the latter formed by adding special (auxiliary) verbs to the simple tenses.

Hapax Logomena
Some suffixes are attested as being attached to only one word and no other instance of attachment is to be found. Similarly, some words are attested only once in the entire extant Old Turkic corpus.

Denominal
The following have been classified by Gerard Clauson as denominal noun suffixes.

Deverbal
The following have been classified by Gerard Clauson as deverbal suffixes.

Literary works
Yenisei Inscriptions (8-10th centuries CE) - a group of texts in Old Turkic from Yenisei River basin.
Uyuk-Tarlak inscription (date unknown) by an unknown writer (in Yenisei Kyrgyz)
Elegest inscription (date unknown) by an unknown writer (in Yenisei Kyrgyz)
Orkhon Inscriptions (732 and 735) by Yollıg Khagan (in Orkhon Turkic)
Bain Tsokto inscriptions (716) by an unknown writer (in Orkhon Turkic)
Ongin inscription (between 716 and 735) by an unknown writer (in Orkhon Turkic)
Kul-chur inscription (between 723 and 725) a writer called "Ebizter" (in Orkhon Turkic)
Altyn Tamgan Tarhan inscription (724) by an unknown writer (in Orkhon Turkic)
Tariat inscriptions (between 753 and 760) by an unknown writer (in Old Uyghur)
Choiti-Tamir inscriptions (between 753 and 756) by an unknown writer (in Old Uyghur)
Sükhbaatar inscriptions (8th century) by an unknown writer (in Old Uyghur)
Bombogor inscription (8th century) by an unknown writer (in Old Uyghur)
Book of Divination (9th century) by an unknown writer (in Old Uyghur)

See also
Old Turkic script
Proto-Turkic
Orkhon Turkic
Old Uyghur
Karakhanid language

References
Further reading
Noten zu den alttürkischen Inschriften der Mongolei und Sibiriens (1898)
Ö.D. Baatar, Old Turkic Script, Ulan-Baator (2008), ISBN 0-415-08200-5
M. Erdal, Old Turkic word formation: A functional approach to the lexicon, Turcologica, Harassowitz (1991), ISBN 3-447-03084-4.
M. Erdal, Old Turkic, in: The Turkic Languages, eds. L. Johanson & E.A. Csato, Routledge, London (1998), ISBN 978-99929-944-0-5
M. Erdal, A Grammar of Old Turkic, Handbook of Oriental Studies, Section 8 Uralic & Central Asia, Brill, Leiden (2004), ISBN 90-04-10294-9.
Erdal, Marcel (1 January 2004). A Grammar Of Old Turkic. BRILL. ISBN 90-04-10294-9.
L. Johanson, A History of Turkic, in: The Turkic Languages, eds. L. Johanson & E.A. Csato, Routledge, London (1998), ISBN 0-415-08200-5
Talat Tekin, A Grammar of Orkhon Turkic, Uralic and Altaic Series Vol. 69, Indiana University Publications, Mouton and Co. (1968). (review: Gerard Clauson, Bulletin of the School of Oriental and African Studies, University of London, 1969); Routledge Curzon (1997), ISBN 0-7007-0869-3.

External links
Old Turkic inscriptions (with translations into English), reading lessons and tutorials
Turkic Inscriptions of Orkhon Valley (with translations into Turkish)
VATEC, pre-Islamic Old Turkic electronic corpus at uni-frankfurt.de.
A Grammar of Old Turkic by Marcel Erdal
Old Turkic (8th century) funerary inscription (W. Schulze)
Kuli Chor inscription complete text
Tonyukuk inscription complete text
Kul Tigin inscription complete text
Bilge Qaghan inscription complete text
Eletmiš Yabgu (Ongin) inscription complete text
Bayanchur Khan inscription complete text
Ongin inscriptions by Gerard Clauson
Timeline of Turkic Languages (Turkish)