It is shown how a model based on a critical discourse analysis approach, in conjunction with frame analysis, offers a systematic way to analyse fifteenth-century letters. It is argued that in this framework all contexts are “local contexts”. Letters present particular difficulties because they are formulaic in certain parts and expressive in others. It is shown that this characteristic may be exploited to facilitate the identification of expressive text. It is further shown that variation in the formulae has a practical application. The formulaic parts of letters that scribes wrote for Margaret Paston may be compared with letters they wrote for themselves. This provides evidence that Margaret herself was responsible for the wording of her letters.