Contract clauses are a big part of the modern game.
They are a way for a club to sweeten a transfer and generally work to the benefit of the player being signed.
But every now and again a club will insert a contract clause to assert a level of control over a player, to ensure they're kept in check.
At Arsenal Alex Oxlade-Chamberlain had a rather bizarre clause written into his contract.
Alan Gernon explained the clause in his book, The Transfer Market: The Inside Stories.
"Following Alex Oxlade-Chamberlain's move to Arsenal in 2011, Gunners fans became increasingly frustrated with the midfielder's lack of playing time," reads a section.
"He was predominantly used as a substitute, often around the 71-minute mark.
"After his contract details emerged, it all became clear: Arsenal were obliged to pay Southampton £10,000 every time he played 20 minutes or more."
Oxlade-Chamberlain isn't the only footballer to have had an unusual clause written into his contract in recent years.