Relation with other West Germanic languages
In the Middle Ages, a dialect continuum existed between Old Dutch and Old Saxon; this was only recently interrupted by the simultaneous dissemination of standard languages within each nation and the dissolution of folk dialects. Despite sharing some features, a number of disparities separate Old Saxon, Old English, and Old Dutch; one such difference is the Old Dutch utilization of -a as its plural a-stem noun ending, while Old Saxon and Old English employ -as or -os. However, it seems that some Middle Dutch took the Old Saxon a-stem ending from some Middle Low German dialects, as modern Dutch still shows the plural ending -s added to certain words.
Old Saxon (or Old Low German) probably evolved primarily from Ingvaeonic dialects in the West Germanic branch of Proto-Germanic in the 5th century. However, Old Saxon, even if it is considered as an Ingvaeonic language, is not a pure Ingvaeonic dialect as Old Frisian and Old English are, the two latter sharing some other Ingvaeonic characteristics, like the great vowel shift that took place in both Old English and Old Frisian. This, plus the large number of different forms that the language took, often showing different West-Germanic features, led some philologists to mistakenly think that Old Dutch and Old Saxon were variations of the same language, and that Old Saxon was indeed an Istvaeonic language.
Relation to Middle Low German
Old Saxon naturally evolved into Middle Low German over the course of the 11th and 12th century, with a great shift from Latin to Low German writing happening around 1150, so that the development of the language can be traced from that period.
The most striking difference between Middle Low German and Old Saxon is in a feature of speech known as vowel reduction, which took place in the other West Germanic languages and some Scandinavian dialects such as Danish, reducing all unstressed vowels to schwa. Thus, such Old Saxon words like gisprekan (spoken) or dagō (days' – gen. pl.) became gesprēken and dāge.