Splitting of [a] and [a:] in Early Old English

PG OE		Other OG languages	OE
	Æ	Gt þata	þæt ‘that’
		O Icel dagr	dæ ‘day’
a	o, ā	Gt mann(a)	mon ‘man’
		O Icel land	land ‘land’
	A	Gt magan	maan ‘may’
		Gt dagos	daas ‘days’
	æ:	OHG dâr	þǽr ‘there’
a:		OHG slâfen	slǽpan ‘sleep’
	o:	OHG mâno	mōna ‘moon’
		O Icel mánaðr	mōnaþ ‘month’

5.3.1.2. Development of Diphthongs

The Proto-Germanic (PG) diphthongs [ei, ai, iu, eu, au] underwent regular independent changes in Early OE; they took place in all phonetic conditions irrespective of environment. The diphthongs with the i-glide were monophthongized into [i:] and [a:], respectively; the diphthongs in u- were reflected as long diphthongs [io:], [eo:] and [ea:]. (The changes are shown in Table 2.2).

Table 2.2