A quasiperiodic crystal, or, in short, quasicrystal, is a structure that is ordered but not periodic. A quasicrystalline pattern can continuously fill all available space, but it lacks translational symmetry. "Aperiodic mosaics, such as those found in the medieval Islamic mosaics of the Alhambra palace in Spain and the Darb-i Imam shrine in Iran, have helped scientists understand what quasicrystals look like at the atomic level. In those mosaics, as in quasicrystals, the patterns are regular – they follow mathematical rules – but they never repeat themselves." "An intriguing feature of such patterns, [which are] also found in Arab mosaics, is that the mathematical constant known as the Greek letters phi or tau, or the " golden ratio ", occurs over and over again. Underlying it is a sequence worked out by Fibonacci in the 13th century, where each number is the sum of the preceding two."