Using the Bitcoin Sign
The bitcoin sign is a capital B with two vertical strokes through the top and bottom, echoing the dollar and the peso. It is the youngest currency symbol in the Unicode standard: the proposal was accepted in 2015 and the character shipped at U+20BF in Unicode 10.0 in June 2017. Before that, the community improvised with Ƀ, a Latin letter B with a bar, and even the Thai baht sign ฿, both of which still turn up in old posts and profile names.
Where it renders and where it does not
Operating systems only picked the glyph up gradually, so support depends on the age of the device rather than the app. Fonts on Windows 10 from its late-2018 update, macOS and iOS from their 2018 releases, and Android 8 and later include it; anything older shows a hollow box, a question mark or a plain B. E-readers, smart TVs, car displays and many receipt printers still cannot draw it. If the audience is unknown, write the ticker BTC or the ISO-style code XBT next to the figure, which is how exchanges and news wires handle it.
Placement and units
Convention borrows from the dollar: the sign goes before the amount, ₿0.05, with no space. Because whole bitcoins are large sums, most prices are quoted in fractions or in satoshis, where one bitcoin equals one hundred million sats; there is no separate sign for the sat, so people write "10,000 sats" or "₿0.0001". Some writers place the sign after the number, 0.05 ₿, mirroring the euro in continental Europe, and that is understood but less common in English.
Not a registered ISO currency
ISO 4217 does not assign a code to bitcoin, so BTC is a ticker by convention rather than a standard. XBT follows the ISO rule that codes for non-national assets begin with X, the same pattern as XAU for gold, and a few exchanges and Bloomberg use it. Spreadsheets will not treat either as currency automatically; format the column as a number and add the sign manually.
Lookalikes and other coins
The Thai baht sign ฿ has one vertical stroke and a rounder shape, and it belongs to a real national currency, so using it for bitcoin can mislead. Other coins borrowed existing letters rather than waiting for Unicode: Ξ for ether, Ł for litecoin and Ð for dogecoin all render on every device precisely because they are ordinary letters, and none is likely to get a dedicated code point.
In social bios the sign signals crypto content, a wallet link or a payment option, often paired with the euro sign or pound sign in a "we accept" line. Stylised alphabets on the bold fonts page will not restyle it, so keep the ₿ itself in plain text.