<P> The empty site beside the shrine building, the site where the previous shrine once stood and where the next will be built, is called the kodenchi . This area is strewn with large white pebbles and is left totally empty apart from the oi - ya, a small wooden hut containing a wooden pole a little over 2 metres in height called the shin - no - mihashira (new sacred central pole). When a new shrine is built, it is built around the sacred central pole before the removal of the oi - ya, so that the central pole is never seen . The central pole of the old shrine will then have a new oi - ya erected so that the shin - no - mihashira also remains unseen . </P> <P> The erection of a single post in the center of a sacred area strewn with stones represents the form taken by Japanese places of worship in very ancient times; the shin - no - mihashira would thus be the survival of a symbolism from a very primitive symbolism to the present day . </P> <P> The shrine buildings at Naikū and Gekū, as well as the Uji Bridge, are rebuilt every 20 years as a part of the Shinto belief of the death and renewal of nature and the impermanence of all things and as a way of passing building techniques from one generation to the next . The twenty year renewal process is called the Sengu . In August, in a long - standing tradition, the people who live in Ise are allowed to enter the area around the Inner Sanctum of the Naiku as well as the Geku . Some villages drag a wooden carriage laden with white stones up the Isuzu River onto the grounds of the Naiku . Each participant gets two white stones in a white handkerchief and these allow them to place the stones in the area around the Inner Sanctum . Other villages drag a huge wooden car or Noburi Kuruma laden with white stones to the Uji bridge at the entrance of the grounds of the Naiku . Participants receive two white stones which are also placed in the sacred space around the Inner Sanctum . The entire tradition is called Shiraisshiki and it is very colourful with every participant wearing a' happi' coat representing a particular village . The rebuilding of the main shrine takes place on a site adjacent to the old, and each rebuilding alternates between the two sites . The next scheduled rebuilding of Naikū is due in 2033 on the lower, northern site . </P> <P> In the lead - up to the rebuilding of the shrines, a number of festivals are held to mark special events . The Okihiki Festival is held in the spring over two consecutive years and involves people from surrounding towns dragging huge wooden logs through the streets of Ise to Naikū and Gekū . In the lead - up to the 2013 rebuilding, the Okihiki festival was held in 2006 and 2007 . A year after the completion of the Okihiki festival, carpenters begin preparing the wood for its eventual use in the Shrine . </P>

Why is the naiku (inner) shrine at ise rebuilt every twenty years