Outer arrays are covered here: http://www.martinrinehart.com/models/tutorial/tutorial_02.html#precision Inner arrays are covered here: http://www.martinrinehart.com/models/tutorial/tutorial_08.html#bed3 (Inner: page down a couple times, until you see the array of ballusters. Start just above the array with "We are about to use an inner array.") You use an outer array when you make a railing and you know how far apart the ballusters should be. You use an inner array where you can position the first and last, but aren't sure about what spacing will look best.