That's amazing, that's exactly it. Traditionaly made Katanas should always be curved :) It happenes in the proces of making the sword, when cooling it for the first time in water after folding the steel several times. Since the traditional Katana is made from a soft steel core (to make the blade flexible) and a harder steel on the outside (to make it strong and razor sharp) the two metals cool at a different rate (like a bi-metal in a barometer) that causes the blade to curve. That first moment of cooling is when the swordsmith knows wether it's going to be a goood blade or not, it either curves gracefully, or breaks. sigh my head is just full with useless trivia... I bet I could list all 7 wonders of the ancient world as well... let's see, hanging gardens of Babilon, Piramides at Gizeh........