File with a .LCK extension contains information on a blockage of a certain file or database, that can be opened and modified only by a single user at a time. LCK files are utilised usually by applications, therefore their modifying should not be performer by users. It is used in order to control simultaneous access to a given file, that is being modified, by several users.

LCK files prevent accidental data damage or changes saved in the same time. They are labelled „open” status while a file which they secure is being modified and „closed” when some other user opens a given file for reading or modification.