Dualone

Circles and crosses are on nine squared pieces.

They can be arranged to make a square 3×3, which can be viewed as a square 6×6.

The puzzle is to put a pair of circle and cross, on six lines and six rows each.

But this is easy, so we add another condition; put also a pair of circle and cross on two diagonals each.

This will reduce the number of (essential) solutions, from more than 90 to just two!

The idea of this puzzle was become real thing by Puzzlein in Kyoto, who attached its wooden case.

The pieces are very simple, but will show you not-so-simple composition.