Back to back example:

Say I fly from New York to LA all the time, fly out on a Tuesday and back on a Thursday. No Saturday night stay.

Next week I am flying on Sept 3 and 5, the following week on the 10th and 12th.

Ticket number one - Round trip New York to LA flying out on Sept 3, and back on Sept 12

Ticket number two - Round trip from LA to New York, flying out of LA on Sept 5, returning on Sept 10.

Both tickets have a Saturday stay and will be less expensive then if they didn't (assuming I bought in advance and all the rest).

When I fly I use the first coupon of the NY to LA ticket on ticket number one for the tuesday, and the first part of the LA to NY ticket of ticket 2 that Thursday.

The next week I fly out on the return portion of ticket 2 and back to NY on the return portion of ticket 1.

Airlines don't like this, but I wouldn't be stupid enough to do it on the same airline.