National Car Rental has launched its sixth annual "One-Two-Free" customer promotion, running now through December 12. For every two qualifying rentals in the U.S. and Canada, the company's Emerald Club members who register for the promotion earn one free rental day—with no limit to the number of free days they can earn.

Participating members can also earn a free rental day for referring a friend.

"This annual Emerald Club promotion has been extremely popular since we launched the program in 2003, and we're pleased to offer it again as one more way to reward our loyal customers for letting us serve their car rental needs," said Greg Stubblefield, president of National Car Rental."

In keeping with the speed and convenience of the Emerald Club rental experience, National sends participants their free rental days via email.

The past “One-Two-Free” promotions have consistently increased Emerald Club membership and driven year-over-year increases in traffic to the company's nationalcar.com Web site.

Emerald Club customers must register at emeraldclub.com/12FREE in order to be eligible for the promotion. The “One-Two-Free” promotion also waives the enrollment fee for new members to join.

Canadian Emerald Club members are eligible for an identical program called "Rent-Rent-Reward." Details are available at emeraldclub.ca/rentrentreward.