The term Old High German (OHG, German: Althochdeutsch, german abbr. Ahd.) refers to the earliest stage of the German language and it conventionally covers the period from around 500 to 1050. Coherent written texts do not appear until the second half of the 8th century, and some treat the period before 750 as 'prehistoric' and date the start of Old High German proper to 750 for this reason. There are, however, a number of Elder Futhark inscriptions dating to the 6th century (notably the Pforzen buckle), as well as single words and many names found in Latin texts predating the 8th century.

Characteristics
There was no standard or supra-regional variety of Old High German - every text is written in a particular dialect, or in some cases a mixture of dialects. Broadly speaking, the main dialect divisions of Old High German seem to have been similar to those of later periods - they are based on established territorial groupings and the effects of the Second Sound Shift, which have remained influential until the present day. But because the direct evidence for Old High German consists solely of manuscripts produced in a few major ecclesiastical centres, there is no isogloss information of the sort on which modern dialect maps are based. For this reason the OHG dialects may be termed monastery dialects.
The main OHG dialects, with their bishoprics and monasteries:
There are some important differences between the geographical spread of the Old High German dialects and that of Modern German:

Central German

• Middle Franconian: Trier, Echternach, Cologne
  Rhine Franconian: Lorsch, Speyer, Worms, Mainz, Frankfurt
  South Rhine Franconian: Weissenburg im Elsaß
  East Franconian: Fulda, Bamberg, Würzburg
  Thuringian: (no texts)
  West Franconian: conjectural dialect of the Franks in Northern Gaul
  Upper German
  
  
  
  • Alemannic: Murbach, Reichenau, Sankt Gallen. Strasbourg
    Bavarian: Freising, Passau, Regensburg, Augsburg, Ebersberg, Wessobrunn, Tegernsee, Salzburg, Mondsee
    Langobardic: (fragmentary, classification as OHG uncertain)
    no German dialects were spoken east of the Rivers Elbe and Saale - in the OHG period this area was occupied by Slavic peoples since the Völkerwanderung and was not settled by German speakers until the late 10th and the early 11th century
    the Langobardic dialect of the Lombards who invaded Northern Italy in the 6th century is assumed to have been an Upper German dialect, though little evidence of it remains apart from names and individual words in Latin texts, and a few inscriptions Dialects
    The Franks conquered Northern Gaul as far south as the Loire; the linguistic boundary later stabilised approximately along the course of the Maas and Moselle, with Frankish speakers further west being romanised. However Frankish is a special case among the old West Germanic languages, the Frankish tribes build their empire at the same time as the High German consonant shift took place. This meant that the dialects of Frankish in the North of their Empire, the Low Countries, did not shift while the dialects in the South did. The dialects in the south are part of Old High German, the ones in the North are part of Old Dutch.
    With Charlemagne's defeat of the Lombards in 776, nearly all continental Germanic speaking peoples had been incorporated into the Frankish Empire. Thus also bringing all continental West Germanic speakers under Frankish rule. However, since the language of both the administration and the Church was Latin, this unification did not lead to any development of a supra-regional variety of Frankish.
    
    Phonology
    Notes:
    
    It seems likely that all back vowels had front allophones as a result of Umlaut, which were then phonemicized in MHG. There was also a mid-close [e] resulting from the Umlaut of /a/ and /e/.
    It is probable that the short high and mid vowels are lower than their long equivalents, as in Modern German, but this is impossible to establish from the written sources.
    Towards the end of the period, short vowels and then long vowels tended to be replaced, when in unstressed syllables, by <e> spellings, which may have represented [ɛ] or schwa [ə]. Vowels
    OHG diphthongs are indicated by the spellings: <ei>, <ie>, <io>, <iu>, <ou>, <uo>.
    
    Diphthongs
    
    There is wide variation in the consonant systems of the OHG dialects arising mainly from the differing extent to which they are affected by the High German Sound Shift. Precise information about the articulation of consonants is impossible to establish.
    In the plosive and fricative series, where there are two consonants in a cell, the first is fortis the second lenis. The voicing of lenis consonants varied between dialects.
    OHG has long consonants, and the following double consonant spellings indicate not vowel length as in Modern German orthography, but rather genuine double consonants: pp, bb, tt, dd, ck (for /kk/), gg, ff, ss, hh, zz, mm, nn, ll, rr.
    /θ/ changes to /d/ in all dialects during the 9th century. The status in the OHG Tatian (c. 830), reflected in modern OHG dictionaries and glossaries, is that <th> is found in initial position, <d> in other positions.
    It is not clear whether the distribution of palatal and velar allophones /c ~ k/ and /ç ~ x/(before front and back vowels, respectively) found in Modern German was already present in OHG. Consonants
    Further information: Medieval German literature
    The early part of the period saw considerable missionary activity, and by 800 the whole of the Frankish Empire had, in principle, been Christianised. All the manuscripts which contain Old High German texts were written in ecclesiastical scriptoria by scribes whose main task was writing in Latin rather than German. Consequently, the majority of Old High German texts are religious in nature and show strong influence of ecclesiastical Latin on the vocabulary. In fact, most surviving prose texts are translations of Latin originals. Even secular works such as the Hildebrandslied are often preserved only because they were written on spare sheets in religious codices.
    The earliest OHG text is generally taken to be the Abrogans, a Latin-Old High German glossary variously dated between 750 and 780, probably from Reichenau. The 8th century Merseburg Incantations are the only remnant of pre-Christian German literature. The earliest texts not dependent on Latin originals would seem to be the Hildebrandslied and the Wessobrunn Prayer, both recorded in manuscripts of the early 9th Century, though the texts are assumed to derive from earlier copies.
    The Bavarian Muspilli is the sole survivor of what must have been a vast oral tradition. Other important works are the Evangelienbuch (Gospel harmony) of Otfried von Weissenburg, the short but splendid Ludwigslied and the 9th century Georgslied. The boundary to Early Middle High German (from ca. 1050) is not clear-cut. The most impressive example of EMHG literature is the Annolied.
    
    Samples
    
    Middle High German
    Medieval German Literature