From: Equestrian Clothing
A riding stock is an item of equestrian clothing that wraps around the riders neck to offer support, it is done up by first putting the button hole of the stock through the button on the stock shirt at the front then wrapping both ends around the back of the neck and bringing around to the front where both are tied one over the other and then again but in the opposite direction.
The loose ends are then arranged into either a straight line down the front or angled off to the side in different directions. It is secured with a stock pin which does up through the stock and the shirt to keep it in place.
During competition a white stock should be worn with a black or blue jacket and a colored stock should be worn with a tweed jacket.