Dunmore, Thomas; Murray, Scott (2012). Soccer for dummies (2 ed.). Indianapolis: John Wiley & Sons, Inc. p. 71. ISBN9781118510667. Retrieved 21 August 2014.
Dunmore, Thomas; Murray, Scott (2012). Soccer for dummies (2 ed.). Indianapolis: John Wiley & Sons, Inc. p. 78. ISBN978-1-118-51066-7. Retrieved 21 August 2014.