A distinction operated since antiquity is that between proper or primary interjections, characterized by the fact that they cannot fulfill the role of other speech parts, and secondary interjections, derived from the interjectional use of items belonging to other word classes. In Priscian's terminology, the former are called primitivae:

            Diomedes notes that not only adverbs, but also other word class items could represent a source for the provenience of interjections, and even whole phrases can be used interjectionally. All these heterogeneous items, are unified by the same semantic parameter: they express emotions. Elements such as mi, ellum, amabo, nefas, pro nefas, malum, miserum, infandum, all of them having originally a referential meaning, in their interjectional use become affective morphemes. On this basis, both Cledonius and Pompeius propose an extended definition, meant to encompass both primary and derived interjections, as well as interjectional phrases: any linguistic unit that arrives to express, in a direct way, an emotion becomes an interjection.

            A similar position embraces Sergius, which adds to the functional and semantic criteria, the intonational parameter of exclamation:

            For Maximus Victorinus, the intonational criterion has priority: any linguistic unit used at an exclamatory mode, becomes an interjection. The wrong idea that interjection would be characterized necessarily and exclusively by an exclamatory intonation persisted until the XX-th century. Some authors however have attenuated this strong statement, with examples of interjections pronounced with an interrogative or mixed intonation.

            A document from the X-th century, De Arte Smaragdi, classifies interjections, according to their form, in simple and compound. The anonymous author offers a series of examples for both categories and discusses some "difficult" cases. He proposes etymologies for the examples where the compound nature of the interjections has lost its transparency, namely where the constitutive elements are fused in a single formula:

            The anonymous author of the Commentary on Donatus (Commentum einsidlense) comments on the sources of secondary interjections and proposes as decisive feature of interjectional conversion, their specific spontaneous way of production [9]. The anonymous author distinguishes between simple secondary interjections (one-word formulae) and compound secondary interjections (many word formulae).