Made for an XL Twin mattress, so if you have a regular twin mattress there will be a couple inches of space in between the mattress and the sidr lip of the bed... the bed will slide around if your child tends to move around a lot.  We secured it against the wall using a side rail so our 18 month old would not get stuck between the wall and the matress.  The product is a great  find if you have the correct mattress!