Two items of the same kind together
tretis, or treitis, is a doublet of "treaty," which also meant a discourse or account.
In the spectra of the alkali metals each line of the trunk is a doublet, and we may speak of a twin trunk springing out of the same root.
Young, according to which the dark line observed in the centre of each component of the sodium doublet in a Bunsen burner is transparent to a radiation placed behind.
Doublet, Notes sur les oeuvres litteraires de l'empereur Hadrien (Toulouse, 1893); J.