V3/usr/man/man5/wtmp.5

Find at most related files.
including files from this version of Unix. 
.pa 1
.he 'WTMP (V)'3/15/72'WTMP (V)'
.ti 0
NAME		/tmp/wtmp  -- user login history
.sp
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DESCRIPTION	This
file records all logins and logouts.
Its format is exactly like utmp(V) except that
a null user name indicates a logout on the associated
typewriter, and the typewriter name 'x' indicates
that UNIX was rebooted at that point.

Wtmp is maintained by login(I) and init(VII).
Neither of these programs creates the file,
so if it is removed record-keeping is turned off.

This file resides in directory /tmp.
.sp
.ti 0
SEE ALSO	init(VII), login(I), acct(VIII), swtmp(VIII)