قطعة المفتاح في أربعة قطعة Tangram

(النص الأصلي) The Key Piece in Four Piece Tangram

The key piece is this one:



It is quite natural to believe that the orientation of this piece is along the longer side:



But this is usually not the case. This piece is usually placed in a corner:



Once you place this piece correctly, it will be very easy to place the other pieces.