The dialect categorized as Mercian is traditionally understood as corresponding to a dialect of Old English spoken in the Midlands of England—between the Mersey and the Humber in the North and the Thames in the South—under the rule of the Kingdom of Mercia. But Campbell has noted that the name Mercia is “practically without territorial significance,” since the distinguishing features of the Mercian dialect are comprised of the agreement between four major texts. Of these texts, the late tenth-century Rushworth Gloss is part of a manuscript which also contains work by a Northumbrian scribe (History of English 402). The other Mercian texts are from Lichfield and the area east of the Severn Valley. For this reason, Hogg suggests calling this dialect “Lichfield Mercian,” since we do not have enough evidence to suggest that these texts are representative of the whole Midlands (An Introduction to Old English 126).
Phonologically, Mercian is characterized by a process called Second Fronting. As the name implies, this was a continuation of First Fronting, a mutation which took place in the Anglo-Frisian languages as they diverged from West Germanic, by which the long vowel [a:] was fronted to [æ:]; compare Gothic laisjan ‘teach’ to Old English *lārjan > lǣren (Prokosch 2009). Alone of the Old English dialects, Mercian continues this trend, raising the vowels /a/ to /æ/ and /æ/ to /e/ (History of English 403). Mercian is also distinguished by morphological and syntactical features, such as the -u ending for the first person present indicative singular, and the comparative rarity of the negative contraction. (An Introduction to Old English 137).
Whether or not Mercian as we know it was a spoken dialect throughout the English Midlands, it does at least appear to have developed as a “focused” literary language by the early ninth century—well before the development of the West Saxon literary tradition. The influence of this Mercian literary tradition extended beyond the Norman Conquest, as demonstrated by the twelfth century The Life of Saint Chad and the AB literary dialect of Middle English (History of English 404).
Hogg, Richard. An Introduction to Old English. Edinburgh Univ. Press, 2010.
Kemenade, Ans van, and Bettelou Los. Handbook of the History of English. Wiley-Blackwell, 2009.
 Though Hogg considers it doubtful that a regularized form of West Saxon which served as a historical “Standard Old English” existed. (Hogg 401)