The Trait is a chant that is sung in one go, ie not interrupted by a chorus (as the alleluia or the offertory and communion antiphons), so it is a non responsorial chant(today, one would call it a chant without a chorus).This should not be seen as evidence of a particularly slow and even less monotonous performance.
Being a non responsorial chant, it is now demonstrated that it is anterior to this kind of chant, hence anterior to the fifth century, and that it has withstood the later generalization of Alleluias. Traits are thus all to be found in the liturgical season of Lent (and of Ember Days), which is the most anciently organized liturgical time. They were thus preserved from the disposal that the new liturgical genre, the Alleluia, which never managed to enter this sanctuary of the liturgical year, threatened them with.
However, everywhere else, Traits were ousted by the responsorial psalm, the ancestor of our gradual, from the late fourth century, and by the Alleluia, from the sixth century onward. This explains the small number of conserved Traits and their strategic location: Wednesdays, Fridays and Sundays of Lent and Ember Days.
There are only three melodies for Traits: one that develops out of the 8th mode and two out the 2nd mode. This also confirms the considerations on the anteriority of Traits compared to Graduals, Offertories and Alleluias, which are to be found in the 8 modes. The more you advance in time, the more diverse the melodies and the greater the number of modes used. Therefore, the absence of Traits in other modes than the two we just mentioned is a proof of their great antiquity. Their slightly archaic character does not detract from their beauty.
In mass, Traits are the last vestiges of psalmody without refrain. Therefore, they are the liturgical kind contemporary with and immediately posterior to the cantica found for example at the Easter Vigil.
Psalmody without refrain, adorned by the singers; psalms sung in full, such are more or less the specifications of Trait; this also explains certain seemingly excessive parts as the Trait of 1st Sunday of Lent (Psalm 90) or the Trait of Palm Sunday (Psalm 21). It is likely that at the beginning and until the fifth century, at a time when the Roman liturgy had two readings before the Gospel, Traits were cut in two, each part being performed after the corresponding reading.