A ticker symbol is a unique abbreviation used to identify publicly traded shares on a specific stock exchange, providing a shorthand for investors to trade and research securities. The first such symbol was used in 1867 for the Union Pacific Railroad, following the invention of the ticker tape machine by Edward Calahan. Symbol formats vary globally: US companies often use 1-4 letters (e.g., Apple's AAPL or Ford's F), European exchanges typically use three-letter codes (e.g., Unilever's UNA), and Asian markets often use numbers (e.g., HSBC's 5 on the Hong Kong exchange). These symbols can change due to mergers (like Exxon's XON becoming XOM) and are sometimes reused, but they are exchange-dependent, so the exchange must be specified for full clarity. To address this limitation, the International Securities Identifying Number (ISIN) was developed—a 12-character code that uniquely identifies a security globally, independent of the exchange, such as Mercedes-Benz's DE0007100000, which remains the same across 22 different exchanges.