Why does the dollar sign "$" sometimes have one vertical line in the middle and sometimes two vertical lines in the middle?

The variation between the single-stroke ($) and double-stroke ($$) dollar sign is primarily a historical artifact of typographic evolution and regional printing conventions, not a distinction with formal semantic meaning. The single vertical line is the overwhelmingly dominant form in modern usage, seen in digital fonts, handwriting, and most printed material globally. The double-stroke version, often called the "cifrão" when used for currencies like the Portuguese escudo or Brazilian real, is a stylistic variant that emerged from 18th-century manuscript traditions. Its use for the US dollar is now largely archaic, surviving chiefly in specialized or decorative contexts.

The origin of the dollar sign itself is widely debated, with the most plausible theory tracing it to the Spanish American peso, or "piece of eight," which was a de facto global currency in the colonial Americas. The symbol is likely a monogram derived from the abbreviation "P^s" for pesos, where the "P" and "S" were superimposed over time. In this process, the vertical stroke of the "P" would merge with the vertical strokes of the "S," potentially creating a symbol with two vertical lines. Early manuscript records from the 1770s show a symbol with two vertical strokes through an "S." The simplification to a single stroke occurred naturally through rapid handwriting and the practical demands of movable type printing, where a single metal piece was easier to cast and align. Thus, the two forms coexisted as typographic alternatives for centuries, with printers and engravers choosing based on design, tradition, or the constraints of their type cases.

In contemporary practice, the distinction is almost purely aesthetic or traditional within specific domains. The double-stroke dollar sign may still be encountered in formal banking or engraving, on some historic banknotes, or in academic economic texts to evoke a classical tone. However, it carries no different meaning than the single-stroke version regarding currency value or denomination. Its notable official use is for other currencies, such as the former Portuguese escudo, where the cifrão (with two strokes) is placed between the escudo and centavo amounts (e.g., 2$50). For the US dollar, the modern standard is unequivocally the single vertical line, as defined by major style guides and digital character encoding standards like Unicode, which treats both forms as glyph variants of the same character (U+0024). The persistence of the double-stroke form serves as a typographic relic, a reminder of the symbol's complex journey from handwritten mercantile shorthand to a universal icon of currency.