This article investigates the articulation of the thumb in flat handshapes (B handshapes) in Sign Language of the Netherlands. On the basis of phonological models of handshape, the hypothesis was generated that the thumb state is variable and will undergo coarticulatory influences of neighboring signs. This hypothesis was tested by investigating thumb articulation in signs with B handshapes that occur frequently in the Corpus NGT. Manual transcriptions were made of the thumb state in two dimensions and of the spreading of the fingers in a total of 728 tokens of 14 sign types, and likewise for the signs on the left and right of these targets, as produced by 61 signers. Linear mixed-effects regression (LME4) analyses showed a significant prediction of the thumb state in the target sign based on the thumb state in the preceding as well as following neighboring sign. Moreover, the degree of spreading of the other fingers in the target sign also influenced the position of the thumb. We conclude that there is evidence for phonological models of handshapes in sign languages that argue that not all fingers are relevant in all signs. Phonological feature specifications can single out specific fingers as the articulators, leaving other fingers unspecified. We thus argue that the standard term ‘handshape’ is in fact a misnomer, as it is typically not the shape of the whole hand that is specified in the lexicon.