The various semantic values of the suffix -ke in Northern Basque are shown to depend, in the analytic conjugations, (a) on the type of verbal complex - aspectually marked participles plus the auxiliaries izan/°edun vs. aspectually unmarked radicals plus the auxiliaries °edin/°ezan (the synthetic conjugation is ambiguous, since it neutralizes this opposition), and (b) on the “tense” of the inflected auxiliaries (present/past/hypothetical). Depending on the context, three epistemic values can thus be computed: equi-possibility, probability, and prediction - hence a specialized interpretation as an ulteriority marker, i.e. a future tense marker when used with a verb in the present tense. This future value is shown not to have been limited to the Souletin dialect, and to have enabled 16th century Basque to have an analytical “future in the future”. Finally, a pragmatic value, the strong commitment of the speaker, will also be identified.