During mourning flags lower a little down to make room for the flag of death Bashny.Net

Tradition half-mast during the mourning appeared in the 17th century for British sailors. After the battle flag lower a little at a distance equal to the width, since his place was occupied by the invisible flag of death. It emphasizes the majesty and the power of death, as well as its ability to equalize the winners and losers in the battle.

In our time in mourning flag to lower a little arbitrary distance as flagpoles are of different lengths, including short width of the flag.

via factroom.ru