The structure editor, found under the "Special Tools" menu, edits the components that the game's screens are constructed from.

Tiles
Tiles are small graphics with associated "physics" (solid, breakable, and air). Four of these are put together to create a combo.

Combos
Combos are the building block of "structures," or the objects that are used to fill the screen. They can be edited by selecting a tile from the tile selector, then holding shift and using the left mouse button to draw in the combo editor. Any changes made in the combo editor will be reflected in all structures that contain that combo.

Structures
These are the building blocks of screens, each composed of several combos. To edit a structure, first click on the combo editor to select a combo. Then, click in the structure editor to paint with the selected combo. You may also right-click on the structure for a menu that allows changes to be made to the shape of the structure.