Airline free stopover

Many airlines collaborate with their hub city's tourism organization to offer free layovers or stopovers to transiting passengers to promote tourism of their hub. These usually include offering a free tour or overnight accommodation. While each airline has different rules, nearly all require you to be flying on their airline to their hub city (sometimes codeshared flights are allowed) and the connecting flights must be done under a single booking (no split ticketing).

There are two subtly different flavors of this:

• A free stopover means voluntarily extending your stay in the transit hub for the purpose of tourism. Often this is "free" only in the sense that your ticket price does not go up, with free hotels being the exception rather than the rule.
• A layover is a connection (nearly always less than 24 hours, only rarely over 24 hours) where no earlier flights are possible. The industry term is STPC (Stopover Paid by Carrier or Stopover For the Purpose of Connection), which typically means a free hotel arranged by the carrier.

Beware: while this article uses the terms stopover and layover consistently, many other sources do not, and they're treated very differently by airlines.