Google Galaxy Nexus Android mobile technology platform 4.0 Users Guide - Manual (Page 101)

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Secure your phone
Set screen lock
Depending how you use your phone, you may want to set some form of automatic screen lock to prevent
unauthorized access. If a screen lock has been set, the screen locks when your phone's display goes to
sleep.
To adjust your phone's lock settings:
1 From a Home or All Apps screen, touch the Settings icon .
2 Touch Personal > Security > Screen lock.
3 Touch the type of lock you want and follow the onscreen instructions.
If you have previously set a lock, you must enter the pattern, PIN, or password to unlock the lock settings.