The hybrid alphabet that had emerged by about AD 1000 then developed over time to produce the script with which we are familiar today. The alphabet used in England around the year 1000 consisted of A, B, C, D, E, F, G, H, I, K, L, M, N, O, P, Q, R, S, T, V, X, Y, Z in the order they occurred in the Latin alphabet, plus three runic letters on the end. These runes gave the sounds th, wy and eth.
In form the runes were unlike modern letters. The rune for th was written þ, eth was written ð, while wy was written . There were also a number of letter combinations that were often written in distinct form. For instance, ae and oe were written as the two letters run together as æ and œ, while ss was written as .
The runes disappeared from usage gradually and had generally gone by the 15th century. An exception was the use of the þ rune if the sound th fell at the start of a paragraph. The rune gradually came to be written in a form almost indistinguishable from a capital Y. When printing arrived, this rune was represented by a Y. It is still sometimes encountered when writers are trying to be deliberately old fashioned, as in the sign ‘Ye Olde Booke Shoppe’ for instance.
The rune was replaced by uu by about 1300 and by 1600 this had become w. The new letter u appeared around 1440 as the pronunciation of words containing the letter v began to vary. The new letter j first appeared in the 1630s in words borrowed from French.
The ae, oe and ss letter combinations persisted in common usage until the 18th century, but had been abandoned from the regular alphabet by 1820 at the latest. That left the 26 letters that we know today.
Answered by Rupert Matthews, historian and author.