1st supine, the fourth principle part, e.g. amātum (for loving)
2nd supine, e.g. amātū (concerning loving). Eitrem § 145: ‘Supine is actually a verbal noun on -us by the 4th declension. The form has the same meaning as present infinitive active. Supine occurs in two forms: 1st and 2nd supine.
a) 1st supine on -um (actually accusative of direction, as per § 73) denotes the purpose of the movement; it takes the same case as the verb.
b) 2nd supine on -ū is used as an ablātīvus līmitātiōnis (as per § 102) with adjectives like facilis, difficilis, utilis, iucundus.