If a thing Rings True, or has the Ring Of Truth to it, it’s thought to be the real thing, the genuine article, despite possible alternatives. In the old days, coins of the realm were made of pure metals – not the hardwearing alloy of modern currency. Pure metals, such as silver, have a sonorous ring to them when dropped on a counter, so it was possible for a person to tell the difference between a genuine coin and a counterfeit by the ringing sound it made.