If you've ever breezed through an airport security line thanks to TSA PreCheck, you have a Known Traveler Number to thank. If your boarding pass is missing that PreCheck marker, the same little code is usually the culprit.

A KTN is a nine-digit identifier, often a mix of letters and numbers, issued by the U.S. Department of Homeland Security to anyone approved for a Trusted Traveler Program — TSA PreCheck, Global Entry, NEXUS or SENTRI. It's essentially your proof to airlines and the TSA that you've been vetted and are allowed into the expedited security lanes at U.S. airports.

Getting one means applying to one of those programs, which involves paperwork and an in-person interview, often at an airport enrollment center. TSA PreCheck runs $78 for five years. Global Entry costs $120 for five years and throws in fast-track passport control when returning to the U.S. from abroad, plus PreCheck benefits — so frequent international travellers usually get better value there. A number of credit cards will reimburse the fee, too.

Here's the step people miss: approval alone doesn't put PreCheck on your boarding pass. You have to add your KTN to the booking. The easiest fix is to store it in your frequent flyer profile with each airline you use; after that it should apply automatically whenever your loyalty number is on the reservation. Booking as a guest? Enter it in the KTN or 'secure traveler' field during checkout.

Forgot to add it? You can usually pull up your reservation on the airline's site or app, drop the number into the traveler information section, and re-download your boarding pass — or reprint it at a kiosk — to see the PreCheck mark appear.

Lost track of the number itself? Log into the DHS Trusted Traveler dashboard, where your Membership Number (also called PASSID) is your KTN. If you carry a physical Global Entry card, the nine-digit PASSID on the back is it. Another handy trick: check your most-used airline profile, where the number is likely already saved.

For anyone flying regularly in or through the U.S., this is low-effort admin with a big payoff — shorter security lines on every trip, and with Global Entry, a faster re-entry after international flights. Apply once, keep the number in your airline profiles, and the airport hassle largely takes care of itself.