How did money emerge out of barter economy

Money emerged out of the barter economy when goldsmiths kept gold for people and gave them receipts instead. The receipts became the "money".

In the 1600s, a lot of London merchants would place their gold in locked storage rooms of the goldsmiths in the city for preservation; the goldsmiths would give receipts to the merchants. The receipts could also be cashed in for gold, which had real value.