T–V distinction
One common system of honorific speech is T–V distinction. The terms "T-form" and "V-form" to describe the second person pronouns "tu" and "vos", respectively, were introduced by Brown and Gilman, whose 1960 study of them introduced the idea that the use of these forms was governed by "power and solidarity." The Latin "tu" refers to the singular T-form, while the Latin "vos" refers to the V-form, which is usually plural-marked. "Tu" is used to express informality, and in contrast, "vos" is used to express politeness and formality. T–V distinction is characteristic of many Indo-European languages, including Persian, Portuguese, Polish, and Russian, as detailed below.