Throughout your use of the application ChordLab will maintain a lookup history of the chords generated. This history will go back the last 16 chords and can be viewed via the History window. The History window can be displayed by pressing the menu shortcut 'H' on the computer's keyboard or by clicking the history button in the main window's toolbar.
The window itself contains a table with columns for index, title and rating of the visited scales. Clicking any row will display that particular chord in the main window. Doing so does not create new history thus enabling one to freely move back through the generated lookup history.
Clicking any of the circled x'es in the right-most column of the history table will delete that specific entry and move all items below up by one row.
With the 'clean history' button in the lower left of the History window, it features a stylized trash can icon, the history can be reset or cleaned which may be useful for a bunch of reasons like specific research of a group of related chords for example.