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They don't.
The wave in the american flag is TRANSVERSE WAVE
transverse wave
Wave of the Flag - 1955 was released on: USA: 14 May 1955
It doesn't 'wave'. It's suspended from the flag pole from a piece of horizontal wire attached to the top of the flag pole at a right angle.
Waving flag by K'naan
Wave your flag.
The cast of Wave a Red Flag - 2009 includes: Lisa Cromarty Monique Mojica
The way the person chooses to wave the flag.
wave or dance
Wave the Flag
The cast of Look Up and Wave Your Glove - 2005 includes: Tony Huston as Tony Huston Kenny Sterner as Kenny Sterner