The nearest relative of the Khoe family may be the extinct Kwadi language of Angola. This larger group, for which pronouns and some basic vocabulary have been reconstructed, is called Kwadi–Khoe. However, because Kwadi is poorly attested, it is difficult to tell which common words are cognate and which might be loans. Beyond that, the nearest relative may be the Sandawe isolate; the Sandawe pronoun system is very similar to that of Kwadi–Khoe, but there are not enough known correlations for regular sound correspondences to be worked out. However, the relationship has some predictive value, for example if the back-vowel constraint, which operates in the Khoe languages but not in Sandawe, is taken into account.