This structure just above is the classic answer given when students are asked to draw the Lewis dot-diagram for CO. However, there are three more.
This is an interesting resonance that is not discussed in the main resonance page. In these two forms, the pi bond has been rotated 90°. Let us say that one structure has the pi bond in the xy plane. Rotate it into the xz plane. That's the second resonance structure.
So, now we go to the last resonance structure.
Pauling identifies the percentage contribution of each structure as:
|both double bonded||40|