The bathtub plug, out of the drain, threaded at the bottom
Or, if you prefer, rightside up as a side elevation.
My bathtub started to not drain properly, so I went on the internet and discovered that I could find more dirt on politicians that I can on finding an easy method of removing a singular bathtub plug. Yes, a bathtub plug, which is not to be confused by the layperson as being the same as a bathtub drain. BIG DIFFERENCE!!!
The "plug", in this case is no where near as to what the BC Liberals are doing every time they pay advertisers to write about how they have been spending our tax dollars on promoting themselves. Its an absolute "No" on finding a "plug" that shows, and tells a method like a "How does it work" scenario that should be available on the internet.
I can only speak for my own home, which is in its infancy (2001) when compared to our old home (1924) on the same property. By BC Building code laws only new products may be used for toilets (ensuite/bathroom), bathtubs (bathroom), sinks (kitchen), and basins (bathroom). The old tub's stopper was a rubber plug thingy, which was great because with the more volume of water contained in the bathtub, the greater the pressure of the water on the plug.
The knurled part on top is for pulling up and twisting, it has nothing to do with the removal of the plug from the drain. The object in removing the plug is to twist in a counter clockwise position while its almost down, and plugging the drain. Too much force will unscrew the knurled part, too little pressure, and nothing happens. However once the plug is unscrewed, with the knurled part in tact (with a little help from a pair of visegrips) it then becomes a simple matter of getting down to the nitty-gritty work of cleaning that which lies beneath the bathtub plug.