The sources of medieval Offices contain a vast number of characteristics which demand comparative study; consequently, analyses of the repertory necessitate the use of a limited set of data upon which observations can be made. While apparently ‘trivial’ variants may be of limited use at the outset, they contribute to the understanding of the repertory and may provide key information to support the relationships between sources. One of these subtle variants is the filling in of a melodic third by the enclosed pitch. An examination of the instances of filled thirds in a selection of sources of the Office for St Thomas Becket offers evidence for an established source grouping. The author attempts to address the question why these filled thirds would exist in sources of relatively new chant melodies. Finally, proposals for further study of filled thirds and similar variants in the plainchant repertory are outlined.