Questions about what Scotsmen wear under their kilts are as old as kilts themselves. At the same time, the kilt's use as military garb has spread kilt-related traditions and lore around the modern world. The contradictory nature of the kilt (skirt-like in appearance, yet ruggedly masculine), and its exotic provenance (the Scottish highlands) lends itself to speculation and humour.
The first description of Scottish kilts in literature can be found in the Lugaidh O'Clery's 1594 Life of Red Hugh O'Donnell. In it, O'Clery describes Scotsmen whose girdles were "over the loins outside their cloaks." The "belted plaid" he describes is the forbearer of the modern kilt. The kilt is not believed to have been a part of everyday wear for most Scots. It became associated with Scottish nationalism with the rise of Romanticism in the early 19th century.
Tradition has it that Scotsmen do not wear underwear underneath their kilts. Yet, Matthew A.C. Newsome writes that "the truth is that we have absolutely no idea how people put on the belted plaid" in 16th-century Scotland. There are no early written instructions or detailed descriptions of how kilts were to be worn. Many widely accepted traditions surrounding the Scottish kilt arose in the 19th century. Today, many men wear whatever they choose beneath their kilts.
Some speculate that because underwear was not common among Scotsmen during the 16th century, it could not have been worn underneath kilts and therefore would not have been part of the kilt's tradition. At the same time, loincloths have been worn by men for millenniums, and it is hard to imagine that 16th-century Scotsmen were completely unfamiliar with them.
Some believe that the tradition of not wearing underwear beneath Scottish kilts can be attributed to the kilt's heavy construction. At their heaviest weights, kilt fabrics can be between 510 to 624gr. Kilts can keep the upper thighs warm during the winter, thus removing the need to wear thermal underwear. At the same time, the kilt's heavy weight could make wearing underwear uncomfortable during summer months, in spite of its ventilation.
Many military regiments require kilts to be worn without underwear. Thus, wearing a kilt without underwear has become known as "going regimental." Exceptions to this rule may include occasions on which the wearer is playing in the pipe band, taking part in organised sports like Highland games, or attending functions where "ladies" are present.
In 2009, Slanj, a leading Scottish kilt manufacturer, made wearing underwear beneath its rental kilts mandatory due to hygiene concerns. This decision was controversial. Iain Emerson, editor of the Famous Tartan Army Magazine, responded by saying "Scots have proudly gone commando at great victories from Bannockburn in 1314 to Wembley in 1977 and that is not going to change now. Our unique national dress makes us popular all over the world and the idea of wearing pants under kilts would shatter illusions and break women's hearts."