Rugs should always be anchored whether used on a hardwood floor or on top of carpet.
How to lay an area rug over carpet.
Area rugs generally work best over short pile or looped carpet.
On a hard floor an untethered rug is more likely to slide or slip but when layering area rugs over carpet the rug is more likely to wrinkle or ripple.
Likewise if you have a plush style of carpet installed choosing a low profile berber or woven rug is your best bet.
The only difference when the rug is going to be used over a carpet is to make sure that the color of the rug complements the color of the carpet.
You may be laying down an area rug because you can t stand the color of the existing wall to wall carpet such as in a rental unit but trust us it will look much worse if you just throw down a rug.