Is a confirmation number required for checking in to a Southwest Airlines flight?

A passenger should use a confirmation number as well as his first and last name when checking in for a Southwest Airlines flight online. However, the airline allows the passenger to look up his confirmation number if he does not have it available.

If passenger does not have a confirmation number to check in, he can look it up by providing his first and last name, the departure city, the departure date, and the credit card number that purchased the ticket. In addition to checking in online, the passenger can modify existing itineraries and add frequent flyer numbers. A passenger can create an online account to manage travel plans as well.