Here's why:
* Sound: In "duty," the "y" makes a consonant sound similar to "d" as in "do".
* Position: The "y" is followed by a vowel ("u") and is not at the beginning of the word, which often indicates a consonant sound.
While the letter "y" can sometimes act as a vowel (like in "my" or "sky"), in "duty" it's acting as a consonant.