OK... what *is* a cleco?

A cleco is an extremely clever means of holding two (or more) sheets of thin
metal (usually aluminum) together for fitting before ultimately riveting. 

[clecos] Several different size clecos are pictured here

A special pliers-like tool squeezes the spring-loaded cleco, which extends the
three inner barbs, which are then inserted into the drilled hole in the work pieces.  

As the pliers are relaxed, the barbed pieces retract, pulling the pieces tightly 
together. Once the fit is assured, the clecos that are holding the work together 
are gradually replaced by permanent rivets.

[pliers] Cleco pliers

Building an aluminimun aircraft without the use of clecos is unimaginable!

In my humble opinion, the cleco ranks right up there with the zipper and velcro
for ingenious and useful fastener technology.