Underbust corsets focus on the waist and hips. Those with strong steel bones can bring in your waist by five or six inches when laced tightly, and they'll also shape and carefully redistribute so you get a classic hourglass silhouette. An acrylic boned underbust is a more casual choice - these can look great but won't significantly alter your natural figure.
You can choose whichever underbust pattern or style suits your intended use, from 20s vintage through to bridal wear. It's worth being aware of the corset length in relation to your height - shorter women should choose shorter length garments and vice versa - but apart from that it's open season. An underbust corset can be added to an existing outfit if you feel that it hasn't got enough flair to match your personality, or if you feel it's not quite as flattering as it could be. With a plain day dress they're an easy way to add a touch of vintage class, and they can also be worn with pant suits. Wear an underbust corset with a dull business outfit and it won't be so dull any more - it'll look more original, more daring without being more casual or more revealing, and a lot more interesting. With a long skirt and a plain-colored top, an underbust makes an instant and intriguing day dress.