Citron
A source of confusion is that "citron" or similar words in French (and other languages), and English are false friends, as they refer to the lemon. Indeed, into the 16th century, the English name "citron" included the lemon and perhaps the lime as well.
Citrus taxonomy
The trifoliate orange is a cold hardy plant distinguishable by its compound leaves with three leaflets and its deciduous nature, but is close enough to the genus "Citrus" to be used as a rootstock. Swingle moved the trifoliate orange from "Citrus" to its own genus, "Poncirus". It was long thought to be the only member of this genus, until a second species, "Poncirus polyandra", was discovered in Yunnan (China) in the 1980s. Early phylogenetic analysis nested "Poncirus" within the citrus, suggesting they all belong to a single genus, but the genomic sequencing of Wu, "et al.", placed it outside the cluster representing "Citrus", and the authors retained a separate "Poncirus" genus.