How to use jQuery in Magento aka jQuery Compatible Mode

Magento shop is using PrototypeJS library which is registering $ for itself. jQuery library wants to do the same thing, that's why in order to use jQuery in a website which uses PrototypeJS (or similar library) you need to put it into Compatibility Mode.

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Source: http://de77.com/javascript/how-to-use-jquery-in-magento-aka-jquery-compatible-mode