T–V distinction
Zamenhof introduced a singular second-person pronoun "ci", to be used in translations from languages where the T–V distinction was important, but he discouraged its use. He added it in the "Dua Libro" in 1888 clarifying that "this word is only found in the dictionary; in the language itself it is hardly ever used", and excluded it from the list of pronouns in the "Fundamento". To this day, it is standard to use only "vi" regardless of number or formality.