Metathesis is the name for the occurrence of a letter out of order in a word or phrase. One not unusual example in north Britain is "modren" to mean "modern". A more literary example is Shakespeare's fictional character 'Falstaff', whose name is formed by metathesis from a historical person called 'Fastolf'. A thousand years ago, the usual word for a feathered warm-blooded animal that flies was 'brid'; by a process of metathesis, it has come to be known as a 'bird'. The Spoonerism is a special case of metathesis.