This is a really good question. I have to admit that I am baffled by the few designs which use the inverted sesquiplane layout.

Possible advantages I can come up with:

• You can use the same wing design and place the lower wing halves left and right of the fuselage and join the upper wings directly above the fuselage. Now the lower span will be wider by the fuselage width, but you only need one set of jigs.
• The upper wing will not only have less span, but also less chord (now you need two sets of jigs, bummer!). With less chord it also is less in the way of the pilot's field of view. This is only valid for small planes with the pilot seated close to the center of gravity, but will not apply to the large Caproni designs.
• If you put ailerons only on the lower wing, where they have a larger lever arm, the control cables can be run directly between the stick and the bell crank for the aileron, so there is less play in the roll axis controls.

But the disadvantages are also obvious:

• Less bank angle is permissible before you have wing tip contact during landing.
• With more lower wing chord, less ground view is left for the pilot, especially when seated close to the center of gravity.
• Especially important for the adherents of the pendulum fallacy: The inverted sesquiplane is believed to be more stable in roll.

This is a really good question. I have to admit that I am baffled by the few designs which use the inverted sesquiplane layout.

Possible advantages I can come up with:

• You can use the same wing design and place the lower wing halves left and right of the fuselage and join the upper wings directly above the fuselage. Now the lower span will be wider by the width of the fuselage, but you only need one set of jigs.
• The upper wing will not only have less span, but also less chord (now you need two sets of jigs, bummer!). With less chord it also is less in the way of the pilot's field of view, especially in turns. This is only valid for small planes with the pilot seated close to the center of gravity, but will not apply to the large Caproni designs.
• If you put ailerons only on the lower wing, where they have a larger lever arm, the control cables can be run directly between the stick and the bell crank for the aileron, so there is less play in the roll axis controls.

But the disadvantages are also obvious:

• Less bank angle is permissible before you have wing tip contact during landing.
• With more lower wing chord, less ground view is left for the pilot, especially when seated close to the center of gravity.
• Especially important for the adherents of the pendulum fallacy: The sesquiplane is believed to be more stable in roll.