What is a metriacal pattern called when stressed syllable followed by an unstressed syllable?

The metrical pattern of a stressed syllable followed by an unstressed syllable is called iamb.

This is one of the most common metrical patterns in English poetry. For example, the line "Shall I compare thee to a summer's day?" from Shakespeare's Sonnet 18 follows an iambic pattern.

Here's a breakdown of the line:

* Shall (stressed) I (unstressed)

* com (stressed) pare (unstressed)

* thee (stressed) to (unstressed)

* a (unstressed) sum (stressed)

* mer's (unstressed) day (stressed)

You can see that each pair of syllables alternates between stressed and unstressed, creating the iambic rhythm.