Conversation so far:
when is a flag flown at half mast
In the United States, the President can issue an executive order for the flag of the United States to be flown at half-staff upon the death of principal figures.
when did the tradition of lowering the flag at half mast start
The tradition of flying the flag at half-mast began in the 17th century.

Question: why half mast
Self-contained rewrite: why is it lowered to half mast