Each stressed syllable (beat) constitutes a metric foot. There will always be unstressed syllables accompanying those that are stressed to make up a given metric foot.
do NOT go GENtle INto THAT good NIGHT (five beats, every 2nd stressed = iambic pentameter)
twas the NIGHT before CHRISTmas and ALL through the HOUSE (four beats, every 3rd syllable stressed = anapestic tetrameter)
Those are Rising Meters
TYger, TYger, BURNing BRIGHT (4 beats. first, third, fifth, seventh syllables stressed = trochaic tetrameter)
GARdens of BABylon (two beats, first, fourth stressed = dactylic dimeter)
Those are Falling Meters
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