What is the keyboard shortcut to delete photos from albums in iPhoto?

This must be a very simple thing, but I can't find reference to it either in Google or in a search of Apple's website. In iPhoto, I created an album of photos to be put on an SD card. That's easy enough. I'm changing the content of the album, and I want to use the keyboard shortcut. The dropdown menu in iPhoto shows "command" + an x inside a box with an arrow pointing left. What key is the second part of this shortcut referring to?
Thanks,
Dan