Abstract Thursday - Different Viewpoint

Unfortunately I don't know the word for what I've blipped - there must be a name for it * - but it's the bottom of the banister post and this is the view looking up the stairwell. A bit Rothko-ish?

Extra   -   a different angle to show you what I mean.

My vocabulary is obviously lacking. Last night while watching television I heard the word 'mullion' (for the first time in my life, I'm sure) and had to google it. Surprise surprise, it's not something you want to win on the lottery, but a vertical bar between two panes of glass in a window. You I learn something new every day!

* All suggestions welcome, even made-up ones!