ttk present but not found for import

[Rich Shepard]

I've started to learn tkinter as the GUI widget set I'll use from now on.
While ttk-8.6 is installed here (/usr/lib/tk8.6/ttk), I cannot import it:

$ python3
Python 3.6.7 (default, Dec  8 2018, 06:22:57) 
[GCC 5.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import tkinter
>>> import ttk
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'ttk'
>>>

What have I done incorrectly?

TIA,

Rich