Rosin(kraft) paper usually. Easier to work with, boards slide easier when nailing. I prefer the results of kraft tho, I think it gives a slightly 'softer', quieter floor imo.
Years ago at Bruce school in Memphis, we were told off the record that Bruce didn't believe it was all that important to use anything because you were poking thousands of holes through the paper and moisture would wick right up the cleat/staple. Cheap easy insurance tho.
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