Practice Lock is an item usually made by Engineers that could be used to practice Lockpicking skill up to skill level 80 prior to Cataclysm. This was a slow and potentially expensive way to increase your Lockpicking skill. After Cataclysm these items were effectively useless as lockpicking skill increased automatically by leveling and could not be increased independently of leveling.
On September 2, 2007, Blizzard patched-in support for the auction house telling potential buyers whether practice locks have been opened or not. Previous to this, unscrupulous players could auction their used practice locks to be bought by unsuspecting players.
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Practice lock is made by engineers with a skill of 100.