Long queues, being packed cheek-by-jowl with other passengers, hand luggage rulings and restrictions that seem to change with the weather, generalised anxiety about the flight ahead , being barked at by overzealous staff: going through airport security is often a stressful and frustrating experience for the average traveller. But if the prospect of being directed to remove your shoes to pass through the body scanner feels like an affliction, it’s worth taking a look at your boarding pass for a code that could make a significant difference to your airport experience. Passengers flying into or within the US may have the misfortune to discover the letters SSSS on their flight documentation.

It stands for Secondary Security Screening Selection and means you can expect to be subjected to a much more intensive screening. SSSS was introduced by the Transportation Security Administration (TSA) , the branch of the United States Department of Homeland Security (DHS) that has authority over transportation security. The TSA was established in response to the September 11 attacks in New York in 2001 .

Its website states that its “mission” is to “protect the nation’s transportation systems to ensure freedom of movement for people and commerce.” Read more: Why are airport queues so long and what are passengers’ rights if they miss a flight? People selected for SSSS can expect a much more extensive screening, with a full-body pat down, mandatory metal detector and body scanning.