How do I know if my key or accessory will fit?

How do I know if my key or accessory will fit?

Jiesheng hardware COMPACT KEY HOLDER will fit almost any flat key. The posts are 4mm in diameter. We don't recommend putting large, plastic covered car keys inside the COMPACT KEY HOLDER because they do not glide easily. These keys can be attached to the loop piece that comes with each COMPACT KEY HOLDER.