Lockpicks

Lockpicks are a tool used to trip the tumblers of a locking device, such as that it may be opened without a key. These handy though somewhat shady tools first appeared in Ultima III and only again vanished in Ultima IX.

Description
Generally, lockpicks are a boon to those dexterous enough to use them correctly, although the delicate metal is prone to breaking when applied incorrectly or used against locks of such craftsmanship as to prevent picking. In cases of the latter, magical skull keys were once employed to bypass particularly recalcitrant locks. In some rare cases, locks are magically enchanted, and require an appropriate spell to open.

Age of Darkness
In the early days of Sosaria, lockpicks were not widely available, with keys (presumably skeleton keys) filling the same function when it came to closely locked doors and treasure.

By the time of Exodus' incursion into the realm in  Ultima III, lock picks were readily available to those willing to do business with the guild shops of the age, and their availability only increased with time.

Age of Enlightenment
During Lord Blackthorn's reign in  Ultima V, many treasure chests carried by wandering creatures had lockpicks within them. Later still, lockpicks became a common commodity sold by provisioners, with no implications that they were in any way illicit.

Age of Armageddon
Curiously, by the late Age of Armageddon in  Ultima IX, these exceptionally helpful tools vanished from all markets, in spite of the seeming power in this era of their original vendor, the Guild of Thieves.

Keys on Earth
During the altered timeline which Minax engendered on the planet Earth, all purpose "keys" were needed to gain access to a number of important locations. For reasons unknown, such instruments could only be found on the slain corpses of castle and towne guards, which necessitated a certain amount of unprovoked murder on the part of the Stranger in order to complete the game.

Locations

 * Sub article: Locations

Trivia

 * In the Ultima series, as in many fantasy portrayals of lock-picking, it is implied that lockpicks may be used singularly to open doors and chests. In reality, the art of lockpicking also requires a tension wrench to provide a pressure comparable to that of a turning key upon the locking mechanism.
 * In the NES port of  Ultima IV, a multi-use artifact known as the Magic Key replaces the disposable skeleton keys found in the original version of the game.
 * In Ultima VII, a large key may be found in a small hut on an island west of Trinsic that serves to unlock any door or chest on which a standard lockpick could otherwise be used, and is unbreakable.
 *  Ultima Underworld II is the only game in the Ultima series in which non-player characters grow upset when the Avatar picks the locks of containers which they own.
 * In  Ultima VII Part Two, Shamino exclaims that the Avatar will no doubt only use lockpicks for virtuous reasons, as the hero is no common thief. Given the kleptomanical play style of many who enjoyed this game, such comments may come across as somewhat naive on the part of well-traveled ranger. More humorous still, none of the companions featured in Serpent Isle ever take issue with the hero using lockpicks for the sake of rampant and decidedly unvirtuous sprees of burglary.
 * You can't buy any lockpicks on the Serpent Isle, only find them. That means you have to make sure not to break too many of them.