What is the historical origin of the convention to place currency symbols like $ or £ before the numerical amount?

Currency symbols like $ and £ are placed before the number for a couple of reasons rooted in history and habit.

A low‑tech fraud fighter

Back when ledgers and checks were written by hand, putting the currency symbol first created a barrier against sneaky alterations. Without the symbol guarding the left side, a crook could easily turn $1,000 into $7,000 by adding a digit. Tucking the $ (or £) up front made that kind of tampering much harder to hide [1].

Following the pound’s lead

The British pound sign already lived before the numerals when Americans started using the dollar sign. Early US documents wrote “$1” in the same pattern as “£1” [2], so the dollar’s position is essentially borrowed from the older British convention, where the pound sign still comes before the digits with no space or a thin space [3].

Not everyone does it that way

While many currencies keep the symbol before the digits (no space, like £100) [4], plenty of others place it after the number. The euro and the Saudi riyal are two common examples [4], and Middle Eastern currencies often put the Arabic symbol after the amount [5]. That variety shows the prefix rule for $ and £ is a historical custom, not a universal grammar law.