Metal D-rings are often used for really heavy-duty jobs, such as lifting or supporting large amounts of weight or pressure. They are normally made from stainless steel, which is the name for iron alloys that have a minimum of 10.5 percent chromium in them. Other alloys are then added to make the steel harder and stronger. Stainless steel is a popular choice for D-rings, particularly those used with animals, because they will not rust. Other metals are available with varying susceptibility to rusting--this is something to think about when purchasing your D-ring.
Plastic D-rings are light and easy to replace, making them perfect for use in mass production. They are normally made from nylon and are used in clothes and bags for less heavy-duty jobs, such as for straps or belt buckles. Nylon was invented by Dr. Carothers in the 1930s as a substitute for silk being used in World War 2 as the primary material for parachutes and ropes. Since then has been developed and evolved for use in most areas modern life.
The Horse's Bit
The D-ring makes up part of the snaffle bit, most commonly used in English and Western riding. Often manufacturers use O-rings as well for making snaffles, depending on how much pressure you want to put on the horse's mouth. These types of bits are best used on young horses, as they are less severe while they are learning. Using O-rings also means that unsteady riders don't have to worry about losing the effect of the reins through the D-ring.
The D-ring on a saddle is often found on an English-style saddle. The rings are usually attached into the saddle itself, although you can buy D-ring attachments for your saddle. They are often sold by the pair and loop onto the stirrup bars and then extend forward, making it easy to attach things to them, such as the breastplate or saddlebags. They aren't just for attaching saddlebags, though; you also can attach lighter gear such as water bottle holders and camera bags, for easy access while riding.
Clothing and Material Use
D-rings are useful, strong pieces of hardware that can be utilized efficiently in material products. The most common place you've probably stumbled across D-rings is on belt buckles or collar fastenings for your pets. Because the D-ring is easily adjustable but very secure once fastened, it's the most cost-effective way to completely personalize the size of the belt or collar. D-rings also have their uses on bags, often attaching the straps to the main body of the bag to allow flexibility for the user. You can buy D-rings in most hardware and craft stores.