[ULPIANUS libro undecimo ad edictum. ] §50.16.19.prLabeo libro primo praetoris urbani definit, quod quaedam 'agantur', quaedam 'gerantur', quaedam 'contrahantur': et actum quidem generale uerbum esse, siue uerbis siue re quid agatur, ut in stipulatione uel numeratione: contractum autem ultro citroque obligationem, quod Graeci συνάλλαγμα uocant, ueluti emptionem uenditionem, locationem conductionem, societatem: gestum rem significare sine uerbis factam.
[ULPIANUS, in the eleventh book on the Edict.] Labeo, in the first book on the Urban Praetor, defines that some things are 'done', some are 'managed', and some are 'contracted': and indeed, that 'what is done' is a general term, whether something is done by words or by a thing, as in a stipulation or a payment; but 'what is contracted' is a mutual obligation, which the Greeks call synallagma, such as buying and selling, letting and hiring, partnership; on the other hand, 'what is managed' signifies a thing done without words.