OE possessed a well-developed system of diphthongs: falling diphthongs with a closer nucleus (ядро) and more open glide arranged in two symmetrical sets - long and short: [ea:, eo:, ie: ] and [ea, eo, ie ]. Towards the end of OE period some of the diphthongs merged (слились) with monophthongs: all diphthongs were monophthongised before [xt, x't ] and after [sk']; the diphthongs [ie:], [ie] in Late WS fused (слились) with [y:, y] or [i:, i ]. Their further development does not differ from the development of corresponding monophthongs.
In Early ME the remaining (оставшиеся) diphthongs were also contracted (стали) to monophthongs: the long [ea: ] merged with the reflex of OE [æ:] - ME [£:]; the short [ea ] ceased to be distinguished (перестал отличаться) from OE [æ] and became [a ] in ME; the diphthongs [eo:, eo ] - as well as their dialectal variants [io:, io] - fell together with the monophthongs [e:, e, i:, i ].
As a result of these changes the vowel system lost two sets of diphthongs, long and short. In the meantime a new set of diphthongs developed from some sequences (последовательностей) of vowels and consonants due to the vocalisation of OE [j ] and [y ], that is (то есть) to their change into vowels. In Early ME the sounds [j ] and [y ] between and after vowels changed into [i ] and [u] and formed diphthongs together with the preceding vowels. These changes gave rise to two sets of diphthongs: with i-glides and u-glides. The same types of diphthongs appeared also from other sources: the glide -u developed from OE [w] as in OE snaw, which became ME snow [snou ], and before lx] and [l] as in Late ME smaul (alongside (наряду) smal) and taughte (NE snow, small, taught).
