Here's why:
* Iamb: A metrical foot in poetry consisting of an unstressed syllable followed by a stressed syllable (da-DUM).
* Pentameter: A line of poetry consisting of five metrical feet.
So, iambic pentameter is a line of poetry with five iambs, resulting in ten syllables with alternating unstressed and stressed syllables.
Example:
> "But soft, what light through yonder window **breaks?"
> (From Romeo and Juliet by William Shakespeare)
This line follows the iambic pentameter pattern:
* But (unstressed) soft (stressed)
* what (unstressed) light (stressed)
* through (unstressed) yon (stressed)
* der (unstressed) win (stressed)
* dow (unstressed) breaks (stressed)
Iambic pentameter is a very common and often considered a very elegant and natural rhythm in English poetry. It's particularly prevalent in Shakespeare's works.