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Marta is a dual-pane file manager. Most of the time you work with two directory panes placed side-by-side. It allows you to do the basic file operations such as copying or moving files in a fast and efficient way, because you can see both source and target directories, and you can copy or move files directly, without copy-pasting it.
One of the panes is active; it means that you can move its cursor using the arrow keys and navigate through the directory tree. The other pane is inactive. You can switch the active/inactive panes using the Tab key.
The active pane is the source, the other (inactive) pane is usually the target. When you run the Copy action, files from the active pane are copied to the inactive pane.
Marta is keyboard-centric: most of the operations are done using the keyboard. All key bindings are configurable. You can read how to set the hotkeys in the Key bindings section.