These spacers are typically used in high-rise concrete deck construction where you pre-situate your conduit and then pour the concrete around it. When you have two or more pipes coming up through the deck and you need to make sure they stay in alignment with each other so they will properly match-up with standard knockouts on junction boxes, you use either Eyeglasses or Nail On Deck Plates.They both come in two or three hole configurations from 1/2" to 1" rigid but the nail-on deck plates have a greater surface area to contact with the wooden deck and are reported to hold the conduit in place better than the typical eyeglasses.