Undersink tailpiece connection cap

[LetsGoMets!]

Between my sink's tailpiece and the p trap is a piece of pipe which has a 2" connection. The connection is capped but the cap is leaking so I have to replace it. So first, does that piece of pipe have a name, or is that still considered part of the tail piece? Also, does the cap have a specific name? Second, the pipes are brass (I think), so does the material of the cap have to be brass, or can it be something else? Third, any specific tips or warnings when removing and replacing? Do I have to wrap the threads in blue tape as well?

Obviously I'm a newbie with this, but everyone has got to start somewhere. I appreciate any attention folks can give to this.

 

[Geofd]

can you show pics....yes what ever metal it is it MUST be the same.....it will cause a reaction between the 2 metals

 

[LetsGoMets!]

The connection used to receive from the double sink (we replaced it 4 years ago with a single sink). The curved tailpiece and the cap were put in by a licensed plumber. The trap and t connection were from the previous owners.

 

[Geofd]

can you show pics....yes what ever metal it is it MUST be the same.....it will cause a reaction between the 2 metals

that's the same type of connection as above...only that's a solid cap you could change the way its piped or leave it the same....its not piped correctly....but if your not confident...in changing the piping you could just change the cap...just keep in mind that the cap is in the water stream all the time....