Metal connectors on suspenders and other pieces of heavy-duty work clothing are beyond helpful. But you may have noticed that the connectors tend to be D rings and not O rings, like what you find on keychains. D rings have openings that are harder to actually open. They require pulling the ends of the ring away from each other and then forcing the ends closed again, instead of merely threading something between layers of the ring. It may seem baffling to those new to using them why the clothing has D rings in the first place. The shape lends itself very well to applications where certain movements need to be alternately limited and made freer.
Limiting Cord/Strap Movement
When you stick a cord or strap through an O ring, the material can move freely through the ring, for the most part. If you put too much through the ring, the material can bunch up and make movement more difficult, if not impossible. But one cord or chain that fits easily through the O ring will move easily, too.
D rings are different, however, in that any material you place in the ring, if the D ring isn't clamped into a certain position, will pull the corner of the ring down. The corner of the D ring is cramped enough that even small amounts of material will have a harder time moving freely through the ring. If you need to attach a strap or cord that you don't want moving freely through a connecting ring, use a D ring.
Allowing/Limiting Freer Side-to-Side Movement
D rings also have the odd ability to both allow and limit certain types of motion when the ring is clamped into a certain position. Think about D rings that are bolted to the side of a truck bed with the flat side of the D ring against the truck bed wall; the ring can move freely one way, such as up and down, while not moving freely the other way, such as side to side. While clamping down an O ring would prevent some movement, the curve of the O ring would make it harder to effectively clamp down the ring in the first place.
Extra Strength
Because D rings have a flat side, the strap or clamp that attaches the D ring to another surface can be relatively wide, offering more strength. With an O ring, the clamp or connecting material can't be that wide as it won't look that good; plus, it will restrict the movement of the ring in ways you might not want.
As you look for work clothing that has ring connectors, like duty suspenders, look for D rings on the clothing. Those rings will be stronger and better-controlled than O rings, allowing for safer connections that won't break as easily. Reach out to a local clothing supplier that sells duty suspenders with D ring belt keepers to learn more.