# LAYERS AND TOOLS

Our integrated circuits consist of two layers, a metal layer and a silicon layer. There is insulation between the layers, except where they are connected by a via.
There are three groups of tools for working on these layers.
* Metal tool
* Silicon tools
* Via tool
Clicking with the Metal tool or one of the Silicon tools adds material to the corresponding layer. Dragging from one cell to the next creates a connection between them.
To remove material, hold down the Alt key (Option on Macintosh keyboards) and click or drag. Material is removed from the metal or silicon layer depending on the tool in use.
Click with the Via tool to create a connection between the metal and silicon layers (shown as a small circle). Alt-click with the Via tool to remove a via.
Note that the silicon tools will only add silicon to empty cells, they won't replace silicon of a different type that's already there. In contrast, Alt-clicking with any silicon tool will remove any type of silicon.
Holding down the Shift key will constrain your mouse movements to horizontal or vertical and help you draw straight tracks.