These Japanese made anodized aluminum front and back filter stacking caps are used to protect a stack of filters. They provide a very easy and safe way to carry filters around. This is two caps, one for the front of the filter and one for the back of the filter. You can use them on one filter or all of your filters screwed together like a tube. The reason I like these so much is because it makes it so easy to see what filter is what, take it out, use it and then put it back when you are done.