This model is housed in an imposing 18-carat 5N gold case measuring 44 millimetres in diameter and 17 millimetres in height. Its designers placed the tourbillon where the fourth wheel was originally positioned: at \u201c6 o\u2019clock\u201d. The chronograph subdial remains at \u201c12 o\u2019clock\u201d. In the retrograde date display, which stretches along a gentle arc on the left-hand side of the dial, the hand jumps back automatically to the 1st of the month after the 31st of the previous month. The IWC-manufactured 89900 calibre allows the time to be set precisely to the second with the help of the hacking tourbillon. The pallet and escape wheel are made from diamond-coated silicon. This provides an extremely hard surface with good surface slip, reducing friction and therefore wear. This reduced friction also compensates for the increased amount of power consumed by the tourbillon, allowing the original power reserve of 68 hours to be maintained. On the reverse side of this watch, which is water-resistant to 3 bar, the see-through sapphire-glass back offers a fascinating view of the sophisticated design of the 89900 calibre.