Building in SL. Objects can be set to snap to a grid (visible here). X and Y coordinates are horizontal, Z is vertical, with <0, 0, 0> being the southwest corner of the current simulator. The object currently being edited glows. The root primitive glows yellow, child prims glow white. In this case many objects are being edited at the same time. The orange dots are the "editing ray" and come from the avatar's left hand.