Internally Threaded Jewelry
Internally threaded Barbells, so named because the bar has smooth ends with receiving threads tapped into the end of the bar. In internally threaded jewelry, the bead has a receiving tube machined into it (this is referred to as "countersunk"), with a threaded post extending from the center, which mates with the threaded receiving tube on the bar. Being much less likely to cause damage and irritation, especially at the time of piercing, internally threaded jewelry is considered superior to externally threaded jewelry.
Internally threaded jewelry passes smooth metal through the piercing as opposed to narrow metal bands, literally reducing wear and tear on your piercing. External jewelry is much more painful inserted into a new piercing.