| First xorg has three buffers, primary, secondary and clipboard. The | primary gets used in the select/middle mouse paste, while the | clipboard gets used by some programs with ctrl-c/ctrl-v is the | standard. So depending on which buffer you want to paste from adjust | the following.
The easy rule of thumb for keeping the selections straight is that if you just select text it becomes the primary selection (for example, if you triple-click the URL bar in Firefox or sweep out some text in your terminal program) while if you actually do an explicit 'copy' action (eg Ctrl-C or picking Copy from a menu) it becomes the clipboard selection. Most of the time, making something the clipboard selection also makes it the primary selection. Middle mouse button paste usually comes from the primary selection. Explicit Ctrl-v or Paste actions from menus usually come from the clipboard selection. (All of this is implemented by individual programs, so every so often you will stumble over one that has decided to go its own way and do something completely different.) One little gotcha about selections in general: all selections are held by the program that created them, and they vanish if you exit the program. If you select some text in Firefox and then quit Firefox before you paste it into anything, poof, it's gone. This is true even if you used Copy/Ctrl-c. (And if Firefox is being hellaciously slow you may not be able to paste that text you selected in it until it recovers.) - cks