ROOCHES ARE USED TO AFFIX A FLY PLAID (OR AN Irish Brath) to the shoulder of a jacket. Originally, the plaid brooch had a very practical purpose in keeping the upper portion of the belted plaid in place. They vary in style, but to be effective, they should be two or more inches in diameter. A heavy brooch is useful for pulling the front of the plaid down, counterbalancing the weight of the fabric draping down the back.