How are Aluminum Post Extensions used?

Our aluminum post extensions are compatible with our 3" square aluminum posts (Aluminum Standard Frame, Aluminum Budget Frame, etc.). They allow for aluminum posts to stand 8' above the ground by installing 2' into the ground (with cement) and 2' into the post. This item is sold as a set of two.  Here's a link to preview:  http://www.4everproducts.com/aluminum-post-extensions-2-pack/