The individuals given lands in return for service of a knight's fee (or part thereof) inevitably grew older, eventually died and their lands and associated obligations passed on to one or several heirs. He or she may not have been able to perform service in an army or, for whatever other reason, did not want to. So a different approach was developed: The obligation of personal service in an army could be avoided either by - sending and paying for another individual to serve in the army as a substitute for the lord, or, paying a fee to the king's exchequer, theoretically allowing the king to fund a substitute.
This fee was called 'scutage', and became a commonly acceptable alternative. The original expected general purpose of scutage was to fund whatever war in which the king was engaged. However, as more and more scutages were required, they became a convenient way for the king to raise funds for his treasury. Scutages under successive kings Henry II, Richard, and John became more and more frequent, and even became one of the causes of revolt by the Barons in the time of King John.
The exchequer kept careful records of scutages due. The Pipe Rolls contain many entries by the treasurers recording, for example, a certain lord owed scutage from a prior collection, plus the current scutage, less payment made if any, and the remaining amount due carried forward. Earlier scutages often amounted to 20 shillings (or £1) per knight's fee, and later, in King John's time, they grew to two marks (or 26 shillings and eight pence) per fee.