Flooring first.

     Yesterday Rob and I went to the house to work on the piano room, at least that was the plan.  When we arrived, I informed Rob that we needed to put the new 10 foot studs in the ceiling and then install a octagon box for the ceiling light.
      Rob said that I should complete the wiring and the outlet boxes in the wall first, which led to completing the lower third of the kitchen side of the wall, which led to completing the kitchen flooring first.
       You see, when we first began laying down the 3/4" subfloor in the hallway and dining room, we actually started in the corner of the kitchen nearest the stairway to the basement. In this area the subfloor goes under the new hallway/dining room wall we built.  However, as you can see in this photo from 2018, we never finished installing the subfloor at the top of the stairs. There has been a hole here with a board laid over it for two years now.
     

So, the first thing we had to do was to build a "saddle" between the kitchen and the living room for 

the subfloor to attach to. Then we installed the 3/4" subfloor.  Then we installed some 1/2" plywood on top of the subfloor to make the height of the floor match what was in the living room and what will be in the hallway.

Finally, we cut some 3/4" plywood which rested on top of our new plywood floor and was nailed to the wall studs to complete the kitchen side of the wall. 

Once that was done, I began installing joist hangers and collar ties on the existing ceiling joists in the piano room ceiling.  While I was doing that, Rob cut and fit all the 1/2" cement board for the hallway floor in preparation for installing the slate tile .