Log chain theory question

Hello!

As per BOL:

http://msdn.microsoft.com/en-us/library/ms186299(v=sql.105).aspx

begins_log_chain bit 1 - This is the first in a continuous chain of log backups.A log chain begins with the firstlog backup taken after the database is created or when it is switched from the simple to the full or bulk-logged recovery model.

As per http://msdn.microsoft.com/en-us/library/ms190440%28v=sql.105%29.aspx

The Log Chain

A continuous sequence of log backups is called a log chain.A log chain starts with afull backup of the database. Usually, a new log chain is only started when the database is backed up for the first time or after the recovery model is switched from simple recovery to full or bulk-logged recovery.

So what really starts a log chain - a DB or a LOG backup???

Thank you in advance,

Michael