Today's previous post recalled a post
from ten years before yesterday's  date.

The subject of that post was the
Galois tesseract.

Here is a post from ten years before
today's  date. 

The subject of that  post is the Halmos
tombstone:

"The symbol    is used throughout the entire book
in place of such phrases as 'Q.E.D.' or 'This
completes the proof of the theorem' to signal
the end of a proof."

— Measure Theory  (1950)

For exact proportions, click on the tombstone.

For some classic mathematics related
to the proportions, see September 2003.