Emma Stone, an actress who has transcended the Em/Emily/Emma/Emmy confusion. (Lucas Jackson/Reuters)

Has this ever happened to you: You’re watching one of the late-night talk shows and the host notes that “Emma Roberts (or Emma Stone or Emily Browning or Emmy Rossum) is on the show tonight,” which forces you to stop and think “Wait, which one is she again?”

We live in the age of Em, where several similarly named actresses in their 20s have starred in sometimes interchangeable roles. (Frankly, I would not be surprised to find out that both Abbie Cornish and Carey Mulligan were born Emmas, only changing their stage names at the behest of a savvy agent.)

Though Emma Watson and Emma Stone (who stars in “Crazy Stupid Love,” opening Friday) might have transcended the confusion, we still feel obliged to include them in the following field guide to the Ems.

Actress She’s the one from:

Coming up:
Emma Stone, 22

“Easy A,” “Superbad,” “Zombieland”

“Crazy Stupid Love,” “The Help,“The Amazing Spider-Man.”

Emma Watson, 21

The “Harry Potter” movies.

“My Week with Marilyn,” continuing her studies at Oxford University.

Emma Roberts, 20

“Nancy Drew,” “Hotel for Dogs,” “Scream 4.”

Rashida Jones’s “Celeste and Jesse Forever”

Emily Browning, 22

“Sucker Punch,” “Lemony Snicket's A Series of Unfortunate Events”

The erotic thriller “Sleeping Beauty.”
Emily Blunt, 28

“The Devil Wears Prada,” “The Young Victoria,” “The Adjustment Bureau”

Jason Segal’s “The Muppets”; “The Five-Year Engagement,” also with Segal.
Emmy Rossum, 24

“The Day After Tomorrow,” “Phantom of the Opera,” Showtime’s “Shameless.” “Shameless,” Season 2