It is difficult to play the note "E" in the third octave on a flute. The E mechanism is a key system to make that note easier to play. Since the E note is widely used in the beautiful melodies of classical and romantic period works, along with technically demanding modern music, the E mechanism is quite useful.
Yamaha flutes with French-style (open hole) keys that have a model number with the number "7" or "9" as their second digit (ex. YFL-574HCT, YFL-892DH) have the E mechanism as a standard feature.
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