I think I know the type of rug pad you mean. I used to see a lot of them in older homes with older large rugs. The pad was pretty thick, like maybe 3/4"? thick. felted wool-like or , as RugPro says, maybe jute?
Haven't seen them recently - maybe they are no longer available and the older ones had to be replaced as they aged? The new ones are maybe more high tech type engineered thinner 1/4" wool-ish? (mothproof, antibacterial, or ?)
In the past I think I paid more attention to these pads than the *rugs* themselves
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