I am not able to put my led cinema AS the main screen all the icons are on the secondary screen .

The Primary screen is the one that holds the Icon for the Menubar is this display.

If not to your liking, DRAG the icon for the menubar to the display you want to be Primary:

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You can not move the actual Menubar.

You must move the Icon for the Menubar in the Arrange pane.

What adapters are you using to connect each of these displays?

Please be very specific.

I have been using this somewhat more terse explanation after doing some experiments. I think it comes to the same conclusion as yours:

The Dock will be displayed on the specified edge of the Primary (MenuBar) Display EXCEPT when a secondary display extends that edge.

(In which case the dock would be displayed on the specified edge of the Extended Desktop.)

The screen with the menubar on it is the primary screen, as previously mentioned to change the primary screen you drag the menu bar from the picture of one screen to the other.

The rules regarding which screen displays the dock icons is more complex. If you have your dock defined to display on the left hand side of the screen then the left hand side of the left hand most screen is where it will appear regardless of which is the primary screen. Likewise if you define the dock to be displayed on the right of the screen it will be displayed on the right side of the righthand most screen. If you define the dock to be displayed on the bottom of the screen then it will be displayed on the bottom edge of the screen which is arranged to be the lowest of however many screens you have.

In the case of the original post his lowest edge is the left screen and hence with the dock set to be displayed on the bottom it will be displayed on the bottom of the left screen even if he changes which is the primary screen. If however the righthand screen is moved down so its bottom edge is below the lefthand screen then it would be used to display the dock.