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.

The formal upright, as the name suggests, is governed by a number of rigid rules. The trunk must be straight, perfectly tapered, without blemishes - in short pristine. The lowest branch must be either to the left or right, the second to the opposite side and the third to the back.
The informal upright is a looser style with the trunk curving from side to side but the branch arrangement following the same rules as the formal upright.

It is important not to straitjacket a tree into a style or to impose on it one that it does not follow in nature. Ficus trees, for instance, look most natural as informal trees with gently rounded canopies, and one shouldn’t impose a triangular periphery to make a peepal look like a Christmas tree. Ficus also looks natural in the broom style: a straight trunk with an umbrella like periphery made from upward growing branches all emanating from, or near, the same point.

Twin,Triple-trunked and clumps of trees all connected at the roots are easy to grow. Many trees send out suckers from their bases and suitable ones can be permitted to grow as adjuncts to the main trunk. Differences in heights and thickness of the trunks are important, and clashes between branches are avoided by choosing outward growing branches on each tree. The overall periphery is treated as one continuous outline.

Bougainvillea spectabilis, ht 6"

Trunks of old trees in nature are broad and spreading at the base.Young trees will grow straight towards the sky but older trunks will be curved and bent after years of battling wind and storms.
Branches of young trees spring energetically upwards while those of older trees bow downwards with the weight of foliage.
All these elements of age can be replicated on bonsai trees so that they look like venerable survivors.
The best way to thicken the trunk base is to allow a branch at the base to grow disproportionately so that it draws nutrition at the base and thickens it.This sacrificial branch is removed after it has done its duty.
Branches are wired downwards to suggest weight and age. Besides wiring they can be tied downwards or weighted by hanging stones etc. on them.