Apologies for swinging this towards Spanish, but it’s bound to occur in any Romance related thread.
In some regions of the Spanish speaking world, “Vos” is used fairly exclusively where everyone else uses “tú”.
Originally, tú was singular, vos was plural. Then it came to be that vos began to be used in reference to those of higher rank (Namely, the emperor).
So, you have:
Singular: informal - tú, formal - vos
Plural: informal - vos, formal vos
This is like the French usage.
By the 1500’s, vos had been extended so far it became almost the same in use as tù. This is when the forms that became “usted” and “ustedes” were developed. Experimenting happened, with forms like simply “merced” - grace, or “señoría” - lordship, and also “tu merced” - your mercy. Vuestra merced and vuerstra mercedes were the forms that found favor, and these became usted and ustedes.
During the Golden Age and the 18th century, tú and vos were in competition, but were resolved in areas closest in contact with Spain - Mexico, Peru, Bolivia, where they used tú. Those furthest often went with Vos, such as the Rio de la Plata area.
Vuestra merced underwent several forms: vuesarced, voacé, vucé, vuced, vested, but finally were reduced to “usted” and “ustedes”.
So, in Spain, the system came to be:
Singular: informal - tú, formal - usted
Plural: informal - vosotros, formal - ustedes
However, in Latin America and Western Andalucia, ustedes came to be used in favor of vosotros. Tú and vos still compete with each other. so the system is:
Singular: informal - tú/vos, formal - usted
Plural: informal - ustedes, formal - ustedes