In the page “cockney rhyming slang ” they explain us some important principles like:
WHAT IS COCKNEY RHYMING SLANG?
“It is a collection of phrases used by Cockneys and other Londoners”
WHAT IS A COCKNEY ?
“It is someone born within the sound of Bow Bells. (St Mary-le-Bow Church in Cheapside, London)…..The Cockney accent is heard less often in Central London these days but is widely heard in the outer London boroughs…”
WHAT IS RHYMING SLANG ?
“Rhyming Slang phrases are derived from taking an expression which rhymes with a word and then using that expression instead of the word. For example the word “look” rhymes with “butcher’s hook“. In many cases the rhyming word is omitted…”