5/8" inch holes mean that this rack is compatible with many accessories from other companies, since unfortunately synergee isnt offering anything other than j cups. Deep enough for standard sterilite boxes, but not too deep to allow wasted space or items to become lost in the back. If you plug this device into a USB port and configure a software drum kit (in Mainstage or Logic Pro on a Mac, for example), you can use this as a MIDI controller; however, the lack of "velocity sensitivity" is a serious limitation. Follow up: in about 2 months, the soles of these slippers flattened, leaving no cushion btwn. non of my other champs have been like this. Needed these in my laundry room. Very cute and flattering and perfect style for summer. They were just ok, fit my pumps just fine, but the cushion was not very soft or cushy. I am really liking the lower 1" heel too. Also, the controller and battery on the wire is heavy, so depending on what youre doing it may pull down on one side and therefore pull the opposite bud lose in your ear.