The concept of isti'naf (also called i'tinaf and ibtida) is used extensively in al-Farra's commentary on the Qur'an as an explanation for nouns in raf, although according to his grammatical theory, the nouns in question require a concrete operator (e.g. their predicates). It is shown in this paper that the term isti'naf has a contrastive sense in these cases, where it indicates that these nouns are not operated on by any previous constituent. Al-Farra's use of formulae such as raf' 'ala al-isti'naf is explained here by reconstructing an earlier stage, in which isti'naf was indeed regarded as the sole explanation of raf', based on the notion of its default status. These formulae survived in al-Farra's time, although their meaning had altered.