Dashes give more emphasis than parentheses to the material they enclose. Many word-processing programs automatically convert two typed hyphens into a solid dash.

The pleasures of reading itself —who doesn’t remember?— were like those of Christmas cake, a sweet devouring.

– Eudora Welty, “A Sweet Devouring”

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

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.

– Susan Sontag, AIDS and Its Metaphors

Marking a sudden change in tone

New York is a catastrophe —but a magnificent catastrophe.

– Le Corbusier

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

Indicating hesitation in speech

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