An amphibrachic foot (known as an amphibrach) has a short syllable followed by a long syllable followed by a short syllable (SLS or U/U).
Tetrameter is four feet per line.
Incontinent, ugly, destructive and smelly,
The troll is a loathsome and pitiful creature.
It lacks any pleasant or positive feature.
There’s hate in its heart and there’s bile in its belly.
You never should feed it – no, not on your Nelly!
It isn’t a candidate for conservation;
The world would improve with its elimination.
Let’s boot out the troll – go on, give it some welly!
Lawrence Eberhart, Don’t Feed the Troll
A closer look at the feet in this poem
U = short syllable; / = long syllable; | = division between feet