After the Oxford comma debate and the death knell of the period, the latest mark to define and divide us — breaking up our thoughts, adding emphasis to our convictions, alternately vexing and delighting readers — is the em dash.

For some writers, the em dash is a vice that their editors occasionally forgive but more often forbid. It has been duly cast as an alluring alternative to the comma, colon, semicolon and full stop in the “distracted boyfriend” meme.

The longest of the dashes — roughly the length of the letter “M” — the em dash is emphatic, agile and still largely undefined. Sometimes it indicates an afterthought. Other times, it’s a fist pump. You might call it the bad boy, or cool girl, of punctuation. A freewheeling scofflaw. A rebel without a clause.

Martha Nell Smith, a professor of English at the University of Maryland and the author of five books on the poet Emily Dickinson (the original em dash obsessive), said that Dickinson used the dash to “highlight the ambiguity of the written word.”