Traditional English verse is divided into repeated units of stressed and unstressed syllables. Their repetition makes the metre or sound pattern, of the poem. Each unit is called a foot. See for example the 'marching' sound of this nursery rhyme:
- The grand old Duke of York,
- He had ten thousand men.
- He marched them up to the top of the hill
- And he marched them down again.
- And when they were up, they were up;
- And when they were down, they were down.
- But when they were only halfway up,
- They were neither up nor down!
The effect of marching is given by the regular stresses of 'grand', 'Duke', 'York'/'had', 'thou-', 'men' etc, with the unstressed syllables in between. This echoes the effect of the sergeant's 'Left, (pause), Left, (pause), Left, Right, Left'. So there are four stresses in the line, each with an unstressed companion (with some irregular feet containing two very quick unstressed syllables - 'to the top of the hill'). This means that there are four feet to the line, making it a tetrameter.