Thirds and Bounds add guides on top of whatever the comp already has. They aren’t part of the grid, so changing the grid or taking it off leaves them alone. Neither one adds a guide where the comp already has one.
Thirds
Click Thirds for rule of thirds: two vertical and two horizontal guides that split the comp into nine.
Shift-click it for the golden ratio, at 38.2% and 61.8% across and down.
Alt-click it (Option on Mac) for one vertical and one horizontal guide through the center.
Thirds always uses the whole comp, even when the grid sits inside a layer. The tile’s icon changes while you hold Shift or Alt, so you can see which one you’ll get.
Bounds
Select one or more layers and click Bounds. Each layer gets six guides: its left, right, top and bottom edges, and a vertical and a horizontal guide through its center.
- Shift-click Bounds for the four edges only.
- Alt-click it for the two center guides only.
Bounds measures what you see of each layer at the playhead, so rotation, scale and parents count. Cameras and lights are skipped. Line other layers up to the new guides by dragging them until they snap.
If the tile says No layers, nothing usable is selected, or the selected layers have nothing visible at the playhead.
Each click is one undo step, named Griddy: Thirds or Griddy: Bounds.