 The uniform-shaped fully interlocking puzzles, sometimes called "Japanese Style", are the most difficult, because the differences in shapes between pieces can be very subtle.Most jigsaw puzzles are square, rectangular, or round, with edge pieces that have one side that is either straight or smoothly curved to create this shape, plus four corner pieces if the puzzle is square or rectangular.Some jigsaw puzzles have edge pieces that are cut just like all the rest of the interlocking pieces, with no smooth edge, to make them more challenging.