This type of lock. 
has two changes
this is found on
most home security doors.

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This type of lock.  sometimes
have two changes. The other
side may have a turn knob with no
​key hole.

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This type of lock, the
majority of the time have
one change.

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This type of lock.
has one change
There may be one on the other
​side.

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This is the barrel.
The parts inside is what we change.
This is the cheapest option.  The lock that you have may still be in good condition, there is no need to purchase a new lock.
their are locks out there that are very cheap, highly not recommended. If you purchase a good quality lock it will last a long time.

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This type of lock
is for glass doors
​it has two changes.

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This front door has a deadbolt and a key in knob
lock, there are three changes. 

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This type of Glass door lock
​has one change on the outside only.

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