A vase is an interesting accessory. They can be beautiful and finely crafted on their own, but they are made to accommodate another item, namely some sort of flower, plant, or decorative facsimile. In this way the vase on its own is often only half of the soul of the piece.
Some vases are fine standing on their own. Glass vases that are placed in front of a window can cast an attractive light on a room. Vases can also be placed on a mantel, or in a centerpiece, to act as austere watchers over the room.
However any of these applications can be enhanced through the use of a second item. In the case of the glass vases placed before a mirror on a sunny day; this effect can be enhanced by pouring water at varying heights into the vases. This has the effect of creating different optical effects through the use of light and refraction. You can even use different colored waters to make a really interesting effect.
Flowers are probably the most common accompaniment to a vase. Smaller bud vases are sized to hold a single flower, while larger ones are designed for use with whole bouquets. The height and depth of the hole will determine the types of flowers that you can use in any given vase.
The problem with flowers is that they tend to get old and die. They also have to be kept watered on a pretty regular basis. Because of this some people get artificial flowers so their vase wont look so lonely. Other people fill their vases with strands of incense, or large wooden leaves.
Sometimes it is attractive to group vases together in decorative arrangements. You can have a row of vases all in the same color and size, or alternate, creating trailing lines going from tallest to shortest. You can also get creative with color, contrasting various tones one off of another.
With some vases made from natural materials, the vase itself may have unique patterns or colors in the surface. This is particularly true of hardwood, or various marble vases.
The vase is a piece that is a dichotomy unto itself. It is made to contain a centerpiece of some sort, such as plants or flowers, however you also want the vase itself to look good.