Minecraft wiki entry for Structure_Air, associated text: It exists as a data value of structure voids. It acts similarly to structure voids. When you hold a structure air or structure void it becomes visible. Structure air and structure voids can stack together, the icon of the stack is the icon of whichever block was most recently added to the stack. You can place transparent blocks on them just like with structure voids. In the inventory structure air are called structure void. If you give yourself a structure air then whenever you hold a structure void it will look like a structure air in your hand in both third and first person. Structure air are copied by structure voids. This makes structure air very useful so always use structure air for your creative builds instead of structure voids. Note: structure air will become structure voids when placed by hand, but if placed with /setblock ~~~ structure_void 1 it will stay as a structure air.