In contrast to parentheses, dashes give more rather than less emphasis to the material they enclose. A typed dash is made with two hyphens (--) with no spaces before, between, or after. Many word-processing programs will automatically convert two typed hyphens into a solid dash (—).

Inserting a comment

Leeches — yuck — turn out to have valuable medical uses.

Emphasizing explanatory material

Indeed, several of modern India’s greatest scholars — such as the Mughal historian Muzaffar Alam of the University of Chicago — are madrasa graduates.

—WILLIAM DALRYMPLE

A single dash toward the end of a sentence may serve to emphasize the material at the end, to mark a shift in tone or a hesitation in speech, or to summarize or explain what has come before.

Emphasizing material at the end of a sentence

In the twentieth century it has become almost impossible to moralize about epidemics — except those which are transmitted sexually.

Marking a sudden change in tone

New York is a catastrophe — but a magnificent catastrophe.

Indicating hesitation in speech

As the officer approached his car, the driver stammered, “What — what have I done?”

Introducing a summary or explanation

In walking, the average adult person employs a motor mechanism that weighs about eighty pounds — sixty pounds of muscle and twenty pounds of bone.

—EDWIN WAY TEALE