IJ2 is out
 Part II: Working with IJ Up Part II: Working with IJ Sec. 5: Finding Commands 

4 Using Keyboard Shortcuts

You’ll learn more and more shortcut keys as you use ImageJ, because (almost) all shortcuts are listed throughout ImageJ menus. Similarly, in this guide each command has its shortcut key listed on its name (flanked by square brackets). Please note that the notation for these key-bindings is case sensitive, i.e., Shift-modifiers are not explicitly mentioned (a capital A means Shift--A) and assumes that Require control key for shortcuts in EditOptionsMisc…↓ is unchecked (i.e., except when using the IJ Editor↓ or the Text Tool↓, you won’t have to hold down the Control key to use menu shortcuts). For example, the command EditInvert [I]↓ can be evoked by Shift I or Ctrl Shift I if Require control key for shortcuts is checked. The full list of ImageJ shortcuts (see Keyboard Shortcuts↓) can be retrieved at any time using the PluginsUtilitiesList Shortcuts…↓ command.
There are three modifier keys in ImageJ:
Control (Command Key on Apple keyboards) Denoted by ‘Ctrl’[?] or Ctrl in this document. Although a control key is typically present on Apple keyboards, on a Macintosh computer running ImageJ the Command key ⌘  Cmd replaces the functionality of the Control key of other operating systems. For sake of simplification, ‘Ctrl’ will always refer to both throughout this guide.
Shift Denoted by ‘Shift’[?] or Shift in this document.
Alt Denoted by ‘Alt’[?] or Alt in this document. This is also the ‘Option’ or ‘Meta’ key on many keyboards. In ImageJ, it is also used to type special unit symbols such as μ (AltM) or Å (AltShiftA).
2 Frontmost Window and Window Activation
In ImageJ, all operations are performed on the active (frontmost) image (which has its title bar highlighted). If a window is already open it will activate when its opening command is re-run, e.g., if the B&C window is already opened (ImageAdjustBrightness/Contrast… [C]↓), pressing its keyboard shortcut ( Shift C ) will activate it.
Pressing Enter on any image will bring the Main ImageJ window↓ to the foreground. In addition, it is also possible to permanently place the main window above all other windows (see Floating Behavior of Main Window↓).
 Part II: Working with IJ Up Part II: Working with IJ Sec. 5: Finding Commands