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Here's an example of the type of shim I'm referring to:

enter image description here

They are sold marketed for various uses - tile spacers, leveling window or door frames, etc. They are non-tapered.

Its not clear to me why the specific horseshoe shape is beneficial. Why not just a simpler flat piece of plastic? Or a more conventional circular "washer" shape if a central open area is needed?

If anything due to the thinness of the material, the ones I've used (1/8" and 1/16") are pretty fragile, which is a slight disadvantage of the shape. (ie it covers a large surface area without commensurate strength).

I assume these were invented for some specific applications ... what are they?

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    What a coincident, I just saw a package of those and purchased a set, First I thought why not just continue using/making wood shims. Then I thought about it. These shims are uniform, won't compress like wood, easier to hammer in, etc. Then I was thinking the same as you, why the U-shape, then a light bulb, it makes it easier to insert when you have a rod/nail between in the middle. You hammer in from the side that gives you the advantage to by passing the nail/object. – Programmer66 Jun 4 '20 at 23:03
  • it may be a marketing strategy ... a product that stands out – jsotola Jun 5 '20 at 7:07
  • Interesting... I've never seen one of these in my life. #TIL – FreeMan Jun 5 '20 at 18:40
  • probably invented by Duncan ⊃∪∩⊂⩍∩ – Jasen Jun 6 '20 at 4:52
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I used these extensively in my kitchen remodel. Wood shims always seem to split on me. And, since I live in a humid area, wood shims disintegrate over time. Many of my original base cabinets have settled leaving a gap between the counter and the backsplash.

This is how I assumed that they should be used. When hanging wall cabinets, you loosely tape them to the wall (open side down) where you think that the screw will be installed. The screw goes inside the horseshoe, near the top. The shim is now captive, the pressure should hold it, but if it doesn't the screw won't let it fall out.

Why not round? Probably so you can insert them after the screw if necessary.

The horseshoe shims that I used were slightly different, they had flexible tabs so they could be stacked.

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I ran into this video: https://youtu.be/uGjY26i6xvU which shows a builder dropping a shim on a screw used to attach something to a wall. Alternatives here would be a solid shim (has to be placed on one side of the screw, making things uneven, or requires 2 shims) or a circle (harder to place correctly, doesn't give additional benefit). The horseshoe shape allows it to be placed while the object is only loosely attached to the wall. It will be held in place by the screw so there's no need to hold it in place.

Another way to think of these is that they aren't a single shim; they're 2 shims joined together for ease of installation. Any time you need to screw two things together, but still keep them from touching, you can just use one of these at each screw location and get good results.

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  1. Bypass nails/obstacles
  2. Material wise they will be more efficient with more support area per unit of material.
  3. It is easier to hit and direct a horseshoe with a hammer than a circle - more directional control when placing into tight opening.
  4. Under a window where the window may allow water to leak on the rough opening sill if you install the open end on the low side of a positive drainage plane any water that leaks into the open area can flow out.
  5. These shims are often trimmed, it is likely easier to trim the ends of the horseshoe than a full side of a circle.
  6. Handling/Gripping a horseshoe is easier than a washer, I feel like you can have a better grip on less of the material. In tile application they can end up stuck and you want to be able to get a good hole on the portion that is sticking out so you can pull them.
  7. The horseshoe shape is good luck and nailed to the wall of a house is supposed to keep the devil out. Can't hurt to have some good luck in your build.
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    Re your point 2 : those horseshoe have more material than a complete circle... – Solar Mike Jun 5 '20 at 7:41

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