Early changes of dental fricatives: English and Frisian compared

Dental fricatives [θ ð] develop along similar lines in English and Frisian throughout
most of the Middle Ages. The consonants were retained in about equal measure, but
alterations occurred when next to other consonants. A way of explaining the changes
in both languages is by invoking complexity of articulation, a notion that finds empirical support. The parallel developments of English and Frisian undermine the idea that Old English evolved differently from other Old Germanic languages during its earliest stages. However, from the late fourteenth century, Frisian took on a different trajectory of change due to new social circumstances connected with increased language contact and bilingualism, especially with Dutch and Low German.