Cadillac 1999 Eldorado Owner's Manual page 59

Ignition Key
The square key is for the ignition only. It has a
resistor pellet which is part of the vehicle's
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PASS Key II system.
The oval key is for the doors and all other locks.
Door/Locks Key
When a new vehicle is delivered, the dealer removes the
plug from the door key. The plug has a code on it that
tells the dealer or a qualified locksmith how to make
extra keys. The ignition key has a bar code tag attached
to it rather than a knock out plug. Your dealer or
qualified locksmith can make extra ignition keys by
reading the bar code tag.
There are 15 alternative ignition PASS
to help discourage theft. Keep the bar code tag and the
door key plugs in a safe place. If you lose your keys,
you will be able to have new ones made using the plug
or bar code tag.
NOTICE:
Your vehicle has a number of new features
that can help prevent theft. You can have a
lot of trouble getting into your vehicle if you
ever lock your keys inside and you may have to
damage your vehicle to get in. Be sure you have
extra keys.
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Key II blanks
2-3