Page "Tabard" Paragraph 6

In the late Middle Ages tabards, now open at the sides and so usually belted, were worn by knights over their armour, and usually emblazoned with their arms ( though sometimes worn plain ).

The Oxford English dictionary first records this use in English in 1450.

In this meaning they were apparently distinguished from surcoats by being open at the sides, and by being shorter.

These became an important means of battlefield identification with the development of plate armor as the use of shields declined.