The shadow of mortality that has passed over Emperor Akihito - he is suffering from prostate cancer - has turned the minds of Japanese people to that rare event, a succession. Since the Meiji Restoration of 1868, when the shogunate collapsed and the emperor was plucked from relative political obscurity in Kyoto to reside in Tokyo, there have only been four emperors.
Japan's royal family claims lineage back to the first emperor, Jimmu (about 650 BC), and is the world's oldest hereditary monarchy. Despite this long lineage, succession was never a problem, due mainly to the system of concubinage which was only abolished in 1926, the year Akihito's father, Hirohito, became emperor.
In 1945, the Americans realised that this system had produced a number of possible competing claimants to the throne. This fear resulted in the Imperial Household Law, introduced in 1948, which limited the succession to male descendants of the emperor, Hirohito.
Emperor Akihito's son, Prince Naruhito, will succeed his father, but the current law will prevent his daughter, one-year-old Princess Aiko, from ascending to the throne. Moves are afoot to change this law in the event that no male heir is born to Prince Naruhito, which would allow Princess Aiko to become empress.
For many Japanese, the only succession they recall was Akihito's in 1989. The country was severely disrupted in the months before Hirohito's death. Cabinet ministers cancelled meetings overseas, television programmes were curtailed, festivities called off and the media portrayed the situation in reverential tones.
Japan in 1989 was a far different place. When Hirohito died, mourning for his passing aside, there was an air of unbridled optimism, the foundation of which lay in the country's phenomenal economic power. From the debris of war, the country had rebuilt itself, joined the economic superpowers and was challenging the US for pre-eminence.