Seiza is a means of sitting via kneeling.  Both knees are on the floor, and your posterior sits more or less on your ankles.  It is said that this style of sitting came about in Japan to prevent people from making sudden moves towards the emporer as he'd pass by.  Most people observe it to be rather painful until they get their joints and muscles stretched enough, so it is possible that this "first application" is true.  However, as one gains further experience with sitting this way, it becomes increasingly comfortable.  Many martial arts studied in Japan concentrated on rapid strikes and evasions while sitting in seiza, so the royal utility of sitting in seiza is compromised.

To sit seiza:
* Kneel on the floor, with both knees down.
* Place your posterior over your ankles.
* Make sure that there are 3 fist-widths between your knees.
* Place your hands on your thighs, or cup them under your navel, with thumbs touching.
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Contrast: ZaZen