A sgian dubh is a small, single-edged knife worn as part of traditional Scottish Highland dress. It is worn tucked into the top of the kilt hose with the hilt visible.
Staghorn sgian dubh being worn on the leg
Sgian dubh with and without sheath
Portrait by Henry Raeburn of Alasdair Ranaldson MacDonell of Glengarry in 1812
A dirk is a long-bladed thrusting dagger. Historically, it gained its name from the Highland dirk where it was a personal weapon of officers engaged in naval hand-to-hand combat during the Age of Sail
Sgian dubh
…about 4 inches (10 cm). These knives usually had antler handles, as do many early sgian-dubhs. The larger knife is likely the ancestor of the modern dirk. The sgian-dubh can be seen in portraits of kilted men of the mid-19th century. A portrait by Sir Henry Raeburn of Colonel Alasdair Ranaldson MacDonell…
Scottish dirk, blade by Andrew Boog, Edinburgh, c. 1795, Royal Ontario Museum
Painting of George Gordon, 5th Duke of Gordon (1770–1836) in highland dress.