The em dash (—) is a versatile punctuation mark used mainly to create emphasis or indicate a break in thought or structure within a sentence. Here are the primary uses and guidelines for when to use an em dash:

1. Replace other punctuation marks:
   • Use it in place of commas, parentheses, colons, or semicolons.
   • Particularly effective for drawing attention to important or surprising information.
2. Set off parenthetical information:
   • Em dashes can bracket additional information that is non-essential but adds detail.
   • This usage is stronger and more interruptive than commas or parentheses.
3. Emphasize appositives containing commas:
   • When an appositive (extra explanatory information) includes commas, use em dashes to avoid confusion.
4. Introduce a summary or amplification:
   • Use an em dash to introduce a list or explanation that follows a sentence, as a less formal alternative to a colon.
5. Indicate a sharp turn or interruption in thought:
   • Em dashes can mark sudden breaks or changes in tone, often seen in creative or informal writing.
6. Mark abrupt changes or interrupted speech:
   • Helpful for dialogue when speech is cut off or hesitant.

Stylistic notes:

• Most style guides recommend no spaces before or after the em dash, but some allow a single space. Consistency is key.
• Em dashes are generally considered more informal and attention-grabbing than the punctuation marks they replace, so use them sparingly for strong effect.

Examples:

• He was late to the meeting — again.
• The answer she gave — surprising as it was — changed everything.
• I have three favorite desserts — cake, ice cream, and pie.
• She wasn’t sure what to say—her words caught in her throat.

In sum, em dashes are a flexible punctuation tool to add emphasis, clarify meaning, or create dramatic breaks in writing.