The style rules of bonsai are meant to replicate trees found in nature.
Starting from the formal upright, where the trunk rises in a straight line from the base to the apex like the Deodars of the Himalyas, to the cascade which plunges downwards like a waterfall, as trees growing on steep cliffs will do, the styles approximately describe the angle that the trunk makes with the ground.

At 90 degrees is the perfect vertical of the formal upright. An informal upright will curve up to 15 degrees either way from the vertical. At 30 to 45 degrees from the vertical the tree can properly be called a slanting tree.
Beyond 45 degrees is the half cascade which the Japanese call han-kengai, with the full cascade or kengai rising almost parallel to the ground and then taking a deep plunge to below the base of the pot.

A bonsai can be anything from less than six inches high to five feet or over. Although the techniques to grow them are basically the same, the smallest trees called mame
( pronounced mah-may), need special care because of the miniscule amount of soil available to them. In Delhi they spend the summer on a bed of gravel that is watered twice a day to provide humidity.

Bonsai takes the sting out of growing old— if you age ten years so will your bonsai.

There's no reason to despair if we can't have hundred year old trees.For a Bonsai can be made to look older than its years.
Looking at old trees in nature one is struck first of all by the spreading roots that seem to grasp the earth with such tenacity. The surface roots of a bonsai are selected early. At every repotting they are spread outwards and encouraged to take the trunk base with them so that it too flares outwards.