To pay respect to the Flag of the United States of America when should you place your hand over your heart?
1. when the National Anthem (Star Spangled Banner) is played,
2. when saying the Pledge of Allegiance to the flag
3. when TAPS is played. (See Note 1 below)

When not to place your hand over your heart? 
Playing of America the Beautiful song or other patriotic songs.

What is the hand over the heart flag etiquette?
To salute, all persons come to attention. Those in uniform give the appropriate formal hand salute. Citizens not in uniform salute by placing their right hand over the heart and men with head cover should remove it and hold it to left shoulder, hand over the heart.
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NOTE 1: TAPS is a 24 note bugle call played during patriotic memorial ceremonies and military funerals to signify the end of a soldier’s duty and provide a final farewell. It symbolizes honor, gratitude, and eternal rest. It may also be played nightly at military bases to signify the end of the day.