Egad! What are all these thread libraries? :-)

I'm starting to look at getting my debugger to understand threaded programs,
and I'm
working on a redhat enterprise linux based system.

I see installed on the system a /lib/libpthread.so, /lib/tls/libpthread.so,
and a /usr/lib/nptl/libpthread.so (actually there are more, but these are
the ones that made me curious).

If I build a threaded program on a redhat 8.0 system, and then cart it over
to the enterprise system, ldd shows it loading the /lib/tls/ version.

If I do the same thing with ASSUME_KERNEL=2.2.5, then the same program
loads the /lib/ version.

So now I wonder what the /usr/lib/nptl/ version is and when it will be used?
Is tls just another name for nptl or what?

I did check and all of them are different sizes and have different inodes,
so they aren't just links :-).

Thanks for any confusion you can relieve.