In a series of articles we have looked at individual early Middle English writing systems and explored aspects of multivocal sound/symbol and symbol/sound relationships. This article combines previous observations with new material, and provides insights into the genesis of these relations and how they may interconnect. Since many early Middle English texts survive as copies, not originals, they may give clues to the orthographic systems of their exemplars too.We investigate the 'extensibility' of Litteral and Potestatic Substitution Sets. Writing systems may be economical or prodigal. The 'ideal' economical system would map into a broad phonetic or a phonemic transcription: that is, one 'sound', one symbol. In early Middle English there is no one standard written norm, so there is potentially less restraint on diversity than in standard systems. Further extensibility is built into the system. We show that much of what tends to be dismissed as 'scribal error' rather represents writing praxis no longer familiar to us -flexible matrices of substitution and variation.South of a line running roughly from the Mersey to the Wash, but excluding much of East Anglia, the distinction [between the functions of 'fl' and 'y'] was regularly maintained; north of this line, and also over much of East Anglia, 'fl' and 'y' were represented by the same (usually y-like) symbol. (LALME 3: xv) The figurae and may sometimes appear as end points of a cline: there is no systematic functional distinction made between -shaped, -shaped or formally indeterminate figurae, however many actual shapes the scribe employs (Benskin 1982: 14).There are therefore four types of scribal praxis in relation to 'fl' and 'y':(a) 'southern' / differentiation both in shape and function;b) 'northern' confusion where a single figura, usually -shaped but sometimes -shaped is used for both [T∼D] and [i∼j]; 4 4 The phonetic representation [i] stands for [i(:)], et passim.