This seems to be quite minor detail is very important and greatly affect the appearance. A wrong neckline can make a disproportionate figure and focus on your shortcomings of others, rather than on the merits. Therefore it is necessary to know at least the basic rules so as not to spend money on things that will only spoil your image.
If you have narrow shoulders, or thighs much wider than shoulder width, the ideal option for you would be a cutout pumps. It helps balance the figure, making it more proportionate. Such a notch fit and women with small breasts.
If the shoulders are sloping shape, square neckline will help to hide this shortcoming.
Women with a short neck is better to prefer a plunging neckline. Suitable for both V-shaped and round neckline. They will make the figure more slender and elongated and add visual low growth, the fairer sex. Deep round neckline will accentuate a small chest, visually adding volume to it.
With broad shoulders and you should pay attention to things with a plunging neckline. They will help give the image of femininity and distract attention from the rough lines. These notches better to combine with sleeves type "bat" or raglan.
Clothing that has no straps (corsets, tops) should not buy obese women, as well as having a long neck or ugly shoulder line. It is best to look at things like slender, low young ladies with perfect posture and manicured hands.
Women with magnificent breasts fit with cut things like "heart." Clothing with cut "American" can afford almost all women. Especially successful purchase such things will be for ladies with a magnificent bust. But thin women with sharp shoulders better to abandon such clothing.
For special events and parties would be appropriate evening dress with a plunging neckline at the back. However, it is worth noting that such things fit slim woman who does not have problems with posture. The notch should not be below the waist, otherwise this outfit is too revealing. These dresses should not be worn for corporate events and serious meeting.