Spelling Changes in Middle English

The most conspicuous feature of Late ME texts in comparison with OE texts is the difference in spelling. The written forms of the words in Late ME texts resemble their modern forms, though the pronunciation of the words was different. Before considering the evolution of English sounds one must get acquainted with the system of ME spelling in order to distinguish between sound changes and graphical changes.
In the course of .ME many new devices were introduced into the system of spelling; some of them reflected the sound changes which had been completed or were still in progress in ME; others were graphic replacements of OE letters by new letters and digraphs.
In ME the runic letters passed out of use. Thorn and the crossed d were replaced by the digraph th, which retained the same sound value; the rune “wynn” was displaced by double uu — w.
The ligatures fell into disuse.

.After the period of Anglo-Norman dominance English regained its prestige as the language of writing, though for a long time writing was in the hands of those who had a good knowledge of French. Therefore many innovations in ME spelling reveal an influence of the French scribal tradition. The digraphs ou, ie, and ch, which occurred in many French borrowings and were regularly used in Anglo- Norman texts were adopted as new ways of indicating the sounds

Also
Sh
Dg
Long sounds were indicated by double letters

Some replacements were probably made to avoid confusion of resembling letters: thus o was employed not only for lol but also to indicate short luI alongside the letter a;it happened when a stood close to n, m, or v, for they were all made up of down strokes and were hard to distinguish in a hand-written text. That is how OE munuc became ME monk, though it was pronounced as Imuokl. This replacement was facilitated — ii not caused — by the similar use of the letter o in Anglo-Norman.
The letter y came to be used as an equivalent of i and was evidently preferred when i could be confused with the surrounding letters m, it and others. Probably y acquired the new sound value. Sometimes, however, y, as well as to, were put at the end of a word for purely ornamental reasons, so as to finish the word with a curve.

The letters th and s indicate voiced sounds between vowels, and voiceless sounds initially, finally and next to other voiceless consonants: ME worthy L’wur, esy [‘e:zi], thyng [thing] (NE worthy, easy, thing, sorrow). Note that in ME — unlike OE this rule does not apply to the letter f: it stands for the voiceless f while the voiced [v] is shown by v or u.

As stated above, o usually stands for [u] next to letters whose shape resembles the shape of the letter u, though sometimes even in the same environment it can indicate lol, ci. ME some (‘sum] and mane (‘mo:nel (NE some, moon).
To determine the sound value of o one can look up the origin of the sound in OE or the pronunciation of the word in NE: the sound lul did not change in the transition from OE to ME (the OE for some was sum).

The digraphs ou and ow were interchangeable. Their sound value can be determined either by tracing the words to OE prototypes or by taking into account the modern pronunciation.