The open back unrounded vowel (script a [ɑ]) fronts to the open, front unrounded vowel [a]. The change was insensitive to neighbouring context and length. 

Aitken and Macafee (2004:§3) claim that the vowel is front in early Scots, although texts from the period give no explicit literal evidence to this effect. According to this, OE short /ɑ/ and long /ɑ:/ became OSc front /a/ and /a:/, respectively.  

The evidence for backness of the vowels in OE, according to Aitken and Macafee (2002 §3), is that they both result in the back diphthong /ɑu/ when followed by OE voiced and voiceless velar fricatives ([ɣ] <g> and [x] <h>.)

The evidence for the frontness of the short OSc vowels comes from a subsequent merger of /a/ with /æ/ in favour of /a/, known as SVL. 

Additionally, for the frontness of the long vowels, there is evidence that the Pre-Scots /a:/ came from OF (cf. OSc [ha:t] 'hot', vs. OF [esta:t] 'estate'). For the purposes of AF, /ɑ:/ would merge with this /a:/ in favour of it (meaning of course that both long and short /ɑ/ ceased to feature in Medieval Scots' inventories.)

Following AF, the short /a/ would undergo the contextually-dependent OSL, which in terms of chronology, is evidence that AF preceded later changes such as OSL and PLD. Finally, diachronically, AF is a precursor to the GVS. Unlike Midland and Southern English where the [ɑ] raises along the back dimension, in Scots the [ɑ] fronts to an [a], which then undergoes the GVS by raising along the front dimension.  

See Johnston, 1997: §3.3.1.2(a) for the long vowel.

See CoNE AF and NLAF

A process by which two or more forms of morphologically related items are reduced in favour of a single phonological shape. This is the case, for example, of OE lician [liʧ-iɑn] 'like, inf.' vs. licade [lik-ɑde] 'like, vpt'. where the root morpheme undergoes AL to OSc. <lik-/lyk> [liːk]. Ultimately AL entails the reduction of allomorphy.

See CoNE AL, Hock (1991: §9), Fertig (2013:§5.1).

This is a cover term for the sporadic tendency throughout the history of Scots and English for consonant clusters of almost any sort to simplify. Many simplifications occur as forms of sandhi. Those that occur across word-internal morpheme boundaries are classified in FITS under IS. CS is reserved for morpheme-internal examples, e.g. OE [timber] > OSc [tɪmɪr].

*hj > [ç]. [h] and [j] mutually assimilate giving a palatal fricative, [ç]. The [j] takes on the voicelessness and fricativeness of the [h] and the [h] takes on the palatality of the [j] yielding a single segment. This follows ((YE)) as the second stage (before ((PF))) of the evolution of [ʃ]-initial forms , e.g. sche, sco, of the OE pronoun hēo SHE. Its outputs are spelled in a number of ways including ᵹ(h)-, g(h)- and ȝ-. A similar change happened to produce the initial palatal fricative [ç] in many PDE varieties in words like Hugh, hue, human, humour (also cf. Shuggie for Hughie in MdSc).

t, d delete when second members of morpheme-final clusters. Deletion is promoted when both members of the cluster are coronal, by absolute finality in the phrase or by the presence of an initial consonant in the following word (less so by a following vowel). This change is scattered throughout the texts, never constant. It remains more or less the same in most PDE casual speech, in which its conditioning environments and variable status appear to have changed little if at all from eME times. It is also possible that this may be a scribal error in some cases. In FITS the change can be seen in <fauch> < OE fæxt fight.

Homorganic Lenthening (HL) refers to the lengthening of short vowels before clusters of two consonants with the same place of articulation, the first of which is a sonorant, the second of which is typically a voiced obstruent or nasal. This change is traditionally dated to the second half of the 8^th and first half of the 9^th centuries (Jordan 1934: 39, Minkova 2014: 165; see also Jespersen (DATE: 118-120), Lass (CHEL2, 1999: 71-72) and Luick (DATE: 474-477), and Aitken & MacAfee (2002: 6-9) and Johnston (1997a: 65-66) specifically for accounts of the change in the history of Scots).

The clusters ostensibly involved in HL were: ld (e.g. OE cild 'child'), mb (e.g. OE climban 'climb'), nd (e.g. OE blind 'blind'), ng (e.g. OE lang 'long'), rd (e.g. OE beard 'beard'), rl (e.g. OE eorl 'earl'), rn (e.g. OE bearn 'bairn, child'), rð (e.g. OE eorðe 'earth'). When a third consonant followed, HL was blocked (e.g. OE cildru 'children').

As is argued in CoNE (Lass et al. 2013), traditional descriptions of HL are not sufficient. Firstly, the change is not regular and appears to be lexically conditioned, with different words being affected in different dialects and with vowel length in particular words not matching in sources from different periods (suggesting variable lengthening with one or other variant winning out through time). Secondly, not all vowels were affected equally; this is especially the case for the low vowel a, which was hardly affected by HL (except before ld, on which see further below). Thirdly, the change was more likely before certain consonant clusters than others, with it being most consistent before ld, mb and nd, and with its application before rC clusters being questionable except in a few cases such as beard. Fourthly, evidence for the date of the change is often lacking (since the spelling may not indicate that any change in length has occurred, cf. Modern English wind (n.) and find (v.)).

Differences in the application of HL in the sequences ind and und constitute one of the classic Ribble-Humber Line isoglosses separating far northern English and Scots dialects (with a retained short vowel) and Midland southern English dialects (with a long vowel), as in blind, find and found, ground (Wakelin 1972: 103). But even here the developments are not regular, with the vowel in wind (n.) only lengthening sporadically in English dialects, and the vowels in kind, mind, pound, round and sound being lengthened everywhere.

Lass et al. (2013) note that evidence for HL before ng is slim. Nevertheless, two definite instances occur in Older Scots: laing 'long' (alongside usual lang) and taingis 'tongs' (the long vowel in this word also underlies traditional pronunciations of this word such as [tjɛŋz] in northeast England), though neither is recorded in the FITS database.

Lengthening of a before nd is absent in later records even though Lass et al. (2013) note that it is indicated by Orm; in any case there appears to be no evidence of it in Older Scots. Before ld, as in cold and old, the existence of the rounded vowel in English dialects south of the Ribble-Humber Line can be taken as evidence for HL (as otherwise there is no obvious motivation for the raising and rounding of original a in these words). Lass et al. (2013) note, however, that HL in these words cannot be proven in northern English or Scots dialects, as the resulting forms (e.g. cauld and auld) could have developed from either the original short vowel or from a long (HL) vowel. Nevertheless, the coincidence of the ARR isogloss and the auld/old isogloss in northern England suggests that HL applied in these words in the north too. See Maguire (2020) for further discussion of the development of a before ld in English and Scots.

As noted, the traditional dating for HL is in the Old English period, in the 8^th-9^th centuries. Lass et al. (2013) review the evidence for the dating of HL and conclude that this early date for HL cannot be proven, and that the earliest secure evidence for the change is in Orm, in the late 12^th century. Since HL must precede ARR (given the presence of ARR in words such as comb and old), a change which is probably datable to the 12^th century, we can assume that HL had occurred by at least this date.

*CoNE HL*

A: A context-free fronting of long back high-mid vowels in Pre Scots. Johnston (1997a:69) dates the change to the late thirteenth century. Crucially, NF must follow the completion of Open Syllable Lengthening (OSL) and Homorganic Lengthening (HL). Indeed, Pre-Scots words with short [u] in an open syllable (such as OE abuvan 'above') underwent Short Vowel Lowering (SVL) to [ʊ], OSL to [oː] and, ultimately NF as well (FITS <aboue> [abøːv]). In HL contexts, Pre-Scots [o] undergoes lengthening and fronting (cf. ONhb word > FITS <wurd> [wøːrd]).

An alternative account of the phenomenon comes from Aitken & Macafee (2002:39-45) who argue that NF entailed not only fronting but raising to [yː] as a single process. This argument is based on the apparent merger of the outcome of NF and Pre-Scots [yː], which they give as a pronunciation of Old French borrowings such as sure.

In FITS, the phenomenon is labelled in accord with Johnston's approach. 

Johnston (1997a)

Aitken & Macafee (2002) 

Love

Broom

Ford

O: The essence is this change is a context-free fronting of Pre-Scots [oː] to [øː] in Older Scots (and many Northern Middle English dialects).

Johnston (1997a:69) dates the change to the late thirteenth century. Crucially, NF must follow the completion of Open Syllable Lengthening (OSL) and Homorganic Lengthening (HL). Indeed, Pre-Scots words with short [u] in an open syllable (such as OE abuvan 'above') underwent Short Vowel Lowering (SVL) to [ʊ], OSL to [oː] and, ultimately NF as well (FITS <aboue> [abøːv]). In HL contexts, Pre-Scots [o] undergoes lengthening and fronting (cf. ONhb word > FITS <wurd> [wøːrd]).

The most common spelling of the fronted vowel are <o, oi, oy, u, w, oo, ou, ow> (see Aitken & Macafee 2002:160).

Aitken & Macafee (2002:39-45) argue that NF entailed not only fronting but raising to [yː] as a single process. This argument is based on the apparent merger of the outcome of NF and Pre-Scots [yː], which they give as a pronunciation of Old French borrowings such as sure. Here, we follow previous scholarship on the change which sees the raising of [øː] as a separate, later process with a various dialectal outcomes (cf. Johnston 1997b:465-6).

In other cases the code is used to indicate a suffixed form not attested in OE, which comes up as part of a lexical item in FITS data, such as -th in dearth.

in some cases, this is technically morphologisation ie make~made

A: A process of neutralisation of unstressed vowels via reduction. A process of reduction in which non-stressed vowels reduced to non-peripheral forms, eventually leading to schwa. 

These are cases where the unstressed vowel surfaces as <e, a, o, ou, u, w> (where it surfaces as <i> or <y> this is a result of WVR, forunstressed diphthongs see UDN).

Here we deal only with non-inflectional vowels, though the process is likely similar in inflectional endings.

The spelling of unstressed vowels are not neutralised to the degree we would expect in a non-standard spelling system. Indeed, although phonologically these vowels behave similarly in their distribution and later developments, the fact that scribes are consistent in using one substitution set for some lexical/etymological categories and others for others tends to indicate that they did not perceive them all to be the same 'colourless' vowel. 

Moreover, we propose that there were likely three or four phonetically distinct unstressed vowel areas, determined either by etymology or the surrounding environment.  While these are probably phonemically /ə/, they are likely to have an array of values, phonetically. Our conventional representation for the vowels in ustressed syllables are [ə], [ʊ], [ɐ] and [ɪ]. 

Afore

Candle

Eve

O: These are cases where the unstressed vowel surfaces as <e, a, o, ou, u, w>, where it surfaces as <i> or <y> we add the WVR process as well.

While these are probably phonemically /ə/, they are likely to have an array of values, phonetically. Our conventional representation for the vowels in ustressed syllables are [ə], [ʊ], [ɐ] and [ɪ].