In June 1907 a musical play " The Merry Widow" opened at Daly's theatre in London.
It was performed 778 times.
"Sonia" the leading lady, was played by Lily Elsie a tall elegant actress who, as a result, became what would now be termed a celebrity.
The costumes including her striking black plumed hat were designed by Lucile (Lucy,- Lady Duff Gordon).The "Merry Widow" was a huge popular hit and the fashion for the hat followed the run of the musical.
In the designers own words:
Every woman who wanted to be "in the swim" , had to have a 'Merry Widow Hat'