What is the rhythm of the following line from the start of Byron's Don Juan?
I want a hero: an uncommon want,
Is it iambic or trochaic? It's a tetrameter and not a pentameter that I am aware of. If it's iambic then why so? Please justify - the same if it's trochaic.
Feet are arbitrary concepts in English verse (see this answer for a detailed discussion of the issue), so when you have a line on its own, with no context, it is impossible to say what its scansion ought to be. All you can say in isolation is what the pattern of streses is. In this case I read the line as follows:
x / x / x x x / x /
I want a hero: an uncommon want,
There are various way to turn this into feet. We could apply the rule “each foot ends with a single stress” (like Patrick Gillespie) and get a tetrameter with three iambs and a paeon:
x / x / x x x / x /
I want | a he- | ro: an uncom- | mon want,
or we could apply the rule “each foot starts with a single stress” (like Gerard Manley Hopkins) and get a tetrameter in sprung rhythm with an anacrusis:
x / x / x x x / x /
I | want a | hero: an un- | common | want,
or we could apply the rule “each foot has two syllables” and get a pentameter with four iambs and one pyrrhic foot:
x / x / x x x / x /
I want | a he- | ro: an | uncom- | mon want,
All these are equally good ways to scan the line, considered on its own. But consider the line in the context of the opening stanza:
I want a hero: an uncommon want,
When every year and month sends forth a new one,
Till, after cloying the gazettes with cant,
The age discovers he is not the true one;
Of such as these I should not care to vaunt,
I’ll therefore take our ancient friend Don Juan—
We all have seen him, in the pantomime,
Sent to the devil somewhat ere his time.
All the lines have ten or eleven syllables, and all but two have five stresses, which means that if we adopt the rule “iambic pentameter with usual range of variations”, we can scan every line in a similar and regular way. This is much simpler than the other approaches, which will cause the number of feet to change from line to line in an irregular way. It is also confirmed by the verse form, which is recognizable as ottava rima, and in English this is commonly written in iambic pentameter.
The usual range of variations includes substituion with pyrrhic feet (as in the first line), substitution with trochees, as in the last line of the stanza:
/ x x / x / x / x /
Sent to | the dev- | il some- | what ere | his time.
and ‘feminine’ endings, as in the second, fourth and sixth lines:
x / x / x / x / x / x
When ev- | ery year | and month | sends forth | a new | one,
