Dollar sign

The dollar sign, also known as peso sign, is a symbol consisting of a capital "S" crossed with one or two vertical strokes ($ or depending on typeface), used to indicate the unit of various currencies around the world, including most currencies denominated "peso" and "dollar". The explicitly double-barred sign is called cifrão in Portuguese. 
The sign is also used in several compound currency symbols, such as the Brazilian real (R$) and the Nicaraguan córdoba (C$).
The one- and two-stroke version are often considered mere stylistic (typeface) variants, although in some places and epochs one of them may have been specifically assigned, by law or custom, to a specific currency. The Unicode computer encoding standard defines a single code for both.
In most English-speaking countries that use that symbol, it is placed to the left of the amount specified, e.g. "$1", read as "one dollar".
History.
Use for the Spanish American peso in the late 1700s.
The symbol appears in business correspondence in the 1770s from Spanish America, the early independent U.S., British America and Britain, referring to the Spanish American peso, also known as "Spanish dollar" or "piece of eight" in British America. Those coins provided the model for the currency that the United States adopted in 1792, and for the larger coins of the new Spanish American republics, such as the Mexican peso, Argentine peso, Peruvian real, and Bolivian sol coins.
With the Coinage Act of 1792, the United States Congress created the U.S. dollar, defining it to have "the value of a Spanish milled dollar as the same is now current" but a variety of foreign coins were deemed to be legal tender until the Coinage Act of 1857 ended this status.
The earliest U.S. dollar coins did not have any dollar symbol. The first occurrence in print is claimed to be from 1790s, by a Philadelphia printer Archibald Binny, creator of the Monticello typeface. The $1 United States Note issued by the United States in 1869 included a large symbol consisting of a "U" with the right bar overlapping an "S" like a single-bar dollar sign, as well as a very small double-stroke dollar sign in the legal warning against forgery.
Earlier history of the symbol.
It is still uncertain, however, how the dollar sign came to represent the Spanish American peso. There are currently several competing hypotheses:
Less likely theories.
The following theories seem to have been discredited or contradicted by documentary evidence:
Currencies that use the dollar sign.
As symbol of the currency.
The numerous currencies called "dollar" use the dollar sign to express money amounts. The sign is also generally used for the many currencies called "peso" (except the Philippine peso, which uses the symbol "₱"). Within a country the dollar/peso sign may be used alone. In other cases, and to avoid ambiguity in international usage, it is usually combined with other glyphs, e.g. CA$ or Can$ for Canadian dollar.
The dollar sign, alone or in combination with other glyphs, is or was used to denote several currencies with other names, including:
Prefix or suffix.
In the United States, Mexico, Australia, Argentina, New Zealand, Hong Kong, Pacific Island nations, and English-speaking Canada, the sign is written before the number ("$5"), even though the word is written or spoken after it ("five dollars", "cinco pesos"). In French-speaking Canada, exceptionally, the dollar symbol usually appears after the number, e.g., "". (The cent symbol is written after the number in most countries that use it, e.g., "5¢".)
Use in the Portuguese Empire.
In Portugal, Brazil, and other parts of the Portuguese Empire, the two-stroke variant of the sign (with the name ; () was used as the thousands separator of amounts in the national currency, the real (plural "réis", abbreviated "Rs."): meant "". This usage is attested in 1775, but may be older by a century or more. It is always written with two vertical lines: . It is the official sign of the Cape Verdean escudo (ISO 4217: CVE).
In 1911 Portugal redefined the national currency as the escudo, worth , and divided into 100 ; but the "cifrão" continued to be used as the decimal separator, so that meant or 123 escudos and 50 centavos. This usage ended in 2002 when the country switched to the euro. (A similar scheme to use a letter symbol instead of a decimal point is used by the RKM code in electrical engineering since 1952.)
Cape Verde, a republic and former Portuguese colony, similarly switched from the real to their local escudo and centavos in 1914, and retains the "cifrão" usage as decimals separator as of 2021. Local versions of the Portuguese escudo were for a time created also for other overseas colonies, including East Timor (1958–1975), Portuguese India (1958–1961), Angola (1914–1928 and 1958–1977), Mozambique (1914–1980), Portuguese Guinea (1914–1975), and São Tomé and Príncipe (1914–1977); presumably all using the "cifrão" as decimals separator.
Brazil retained the real and the "cifrão" as thousands separator until 1942, when it switched to the Brazilian cruzeiro, with comma as the decimals separator. The dollar sign, officially with one stroke but often rendered with two, was retained as part of the currency symbol , so one would write for 13 cruzeiros and 50 centavos.
It was formerly used by the Portuguese escudo (ISO: PTE) before its replacement by the euro and by the Portuguese Timor escudo (ISO: TPE) before its replacement by the Indonesian rupiah and the US dollar. In Portuguese and Cape Verdean usage, the is placed as a decimal point between the escudo and values. The name originates in the Arabic (), meaning 'zero'.
One stroke vs. two strokes.
In some places and at some times, the one- and two-stroke variants have been used in the same contexts to distinguish between the U.S. dollar and other local currency, such as the former Portuguese escudo.
However, such usage is not standardized, and furthermore the two versions are generally considered mere graphic variants of the same symbol—a typeface design choice. Computer and typewriter keyboards usually have a single key for that sign, and many character encodings (including ASCII and Unicode) reserve a single numeric code for it. Indeed, dollar signs in the same digital document may be rendered with one or two strokes, if different computer fonts are used. Should that ambiguity be significant, one may use explicit abbreviations (like "" or "$NZ"), or ISO 4217 three-letter currency codes (such as USD and MXN) to distinguish different currencies that use the symbol.
When a specific variant is not mandated by law or custom, the choice is usually a matter of expediency or aesthetic preference. Both versions were used in the US in the 18th century. (An 1861 Civil War-era advertisement depicts the two-stroked symbol as a snake.) The two-stroke version seems to be generally less popular today, though used in some "old-style" fonts like Baskerville.
Use in computer software.
Because of its use in early American computer applications such as business accounting, the dollar sign is almost universally present in computer character sets, and thus has been appropriated for many purposes unrelated to money in programming languages and command languages.
Encoding.
The dollar sign "$" has Unicode code point U+0024 (inherited from ASCII via Latin-1).
There are no separate encodings for one- and two-line variants. The choice is typeface-dependent, they are allographs. However, there are three other code points that originate from other East Asian standards: the Taiwanese small form variant, the CJK fullwidth form, and the Japanese emoji. The glyphs for these code points are typically larger or smaller than the primary code point, but the difference is mostly aesthetic or typographic, and the meanings of the symbols are the same.
However, for usage as the special character in various computing applications (see following sections), U+0024 is typically the only code that is recognized.
Support for the two-line variant varies. the Unicode standard considers the distinction between one- and two-bar dollar signs a stylistic distinction between fonts, and has no separate code point for the . The symbol is not in the October 2019 'pipeline', but appears to be under active consideration. 
The following fonts display a double-bar dollar sign for code point 0024: regular-weight Baskerville, Big Caslon, Bodoni MT, Bradley Hand ITC, Brush Script MT, Garamond, STFangsong, STKaiti, and STSong ($). 
In LaTeX, with the textcomp package installed, the (formula_5) can be input using the command codice_1. 
However, because of font substitution and the lack of a dedicated code point, the author of an electronic document who uses one of these fonts intending to represent a cannot be sure that every reader will see a double-bar glyph rather than the single barred version.
Because of the continued lack of support in Unicode, a single bar dollar sign is frequently employed in its place even for official purposes. Where there is any risk of misunderstanding, the ISO 4217 three letter acronym is used.
Other uses.
The symbol is sometimes used derisively, in place of the letter S, to indicate greed or excess money such as in "Micro$oft", "Di$ney", "Chel$ea" and "GW$"; or supposed overt Americanisation as in "$ky". The dollar sign is also used intentionally to stylize names such as A$AP Rocky, Ke$ha, and Ty Dolla $ign or words such as . In 1872, Ambrose Bierce referred to California governor Leland Stanford as $tealand Landford.
In Scrabble notation, a dollar sign is placed after a word to indicate that it is valid according to the North American word lists, but not according to the British word lists.
A dollar symbol is used as unit of reactivity for a nuclear reactor, being the threshold of slow criticality, meaning a steady reaction rate, while is the threshold of prompt criticality, which means a nuclear excursion or explosion.
The dollar sign was used as a letter in the Turkmen alphabet from 1993 to 1999.
The dollar sign plays an important role in the plot of Ayn Rand's novel "Atlas Shrugged", with the book's radical Free Market activists adopting it as their insignia.