When light infantry, that hitherto had been organised in small units like the free battalions (Freibataillone), became part of the line troops, and the Landwehr, national guards and the like became part of the warfighting army, the term "line regiment" was used to distinguish the standing, active, regular units from the rest.
They usually made up the main body of an army's strength.
In the British Army during the late 19th century and First World War, the Line Cavalry Regiments were the regiments of the Dragoon Guards, Dragoons, Hussars, and Lancers.
[1] In the Napoleonic Wars, the French Army had up to 30 regiments of dragoons which comprised the "cavalry of the line".
[2] This article about a specific military unit is a stub.