HGTV says Contain Yourself: Getting to Know Containers for Flower Arranging
From: DK Books - Flower Arranging
Every flower design must suit its purpose and surroundings. Decide on your container first, before you select the flowers. Consider the size, shape, and color of the container in order to get the right look, and create the right impression. This selection of vases represents the four essential shapes in a designer's collection; with this limited choice you can create a wonderful variety of arrangements and show off the flowers at their best.
This globe shape is good for highlighting the beauty of just a few blooms by curling and swirling them within the curved contours of the bowl. It is also ideal for displaying tall-stemmed hand-tied bouquets that require a dramatic edge.
A square-shaped vase is perfect for displaying a mass of one type of flower, and ideal for smaller, compact arrangements with short-stemmed flowers. The straight sides of this vase put a geometric framework onto the blooms, giving them a modern look.
This vase allows flowers and foliage to fall naturally at pleasing angles to create a fan shape, and it can display a wide variety of flowers clearly in a large arrangement. Its tapered base also limits the spread of their stems, ensuring that any displays requiring an extravagant flourish or an element of drama or impact have added height.
A straight-sided vase can "contain" the flowers within it, so you can use it to create sculptural or uniform arrangements. Its extended height allows displays of tall-stemmed flowers to be well supported so they don't droop, but you can also use it for contemporary designs, such as a compact ball or "bomb" of large, domed flowers that sits on the rim of the vase.