Primary Selection.

The primary selection is silently established by selecting some text in a text field. It can be pasted into another text field simply by using the middle mouse button. Unlike the clipboard, there is no memory region to remember the primary selection other than the selection on the screen itself. Thus, changing the selection immediately overwrites the old one. Analoguous to the clipboard, there is only one primary selection active among all applications on the desktop; selecting text in another application will deselect the text in the current one.