These are generally referred to as carpet to carpet pads.
How to put down a rug pad.
Another alternative would be to use a rug pad which is slightly smaller but the difference should not exceed 12 square inches.
If using the waffle pad style of carpet pad the waffle side of the carpeting pad should be face up for maximum cushioning and it is cut to fit inside the tackless strips with about gap so that the carpet itself can be attached to the tackless strips and the padding does not interfere with the securing of the carpet.
These types of pads are specially designed to help keep an area rug from shifting and rippling over the carpet.
Before you can put down the rug pad you will need to start by clearing out any furniture that can inhibit your full view of the space since having furniture in the way can keep the rug from laying flat.
Smooth down any air bubbles that might be trapped between the pad and the area rug.
There is a non slip rug pad designed for pretty much every use of an area rug including holding an area rug over broadloom.
Use a non slip pad.
Subtract 2 inches from both the length and the width of the rug to get the measurements for the rug pad.
It gives time for the area rug s fibers to relax and for the rug pad to settle on the floor.
Placing the rug pad.
You may use a rug pad of the same size of the rug.
Lay the rug pad out on the floor and measure the length that you determined for the pad down the side of the rug pad.
Clean the whole room to ensure that the pad will stay securely in place.
Cut straight across the pad to cut a piece slightly shorter than the length of the carpet.
You must lay the rug pad at the location where you wish to place the rug.
Wait 1 2 days before trimming.
If you can live with a mismatched area rug and rug pad on the floor for a couple of days i strongly recommend this step.
Using the washi tape as a guide roll out your area rug pad and place the area rug directly on top.