Jewel neck

The neckline is the top edge of a garment that surrounds the neck, especially from the front view. Neckline also refers to the overall line between all the layers of clothing and the neck and shoulders of a person, ignoring the unseen undergarments.

For each garment worn above the waist, the neckline is primarily a style line and may be a boundary for further shaping of the upper edge of a garment with, for example, a collar, cowl, darts or pleats. In that respect it is similar to the waistline and hemline.

A folded turtleneck

T-shirt neckline

Men and women of the Royal Australian Navy in uniform V-necked shirts

Necklines can be grouped into categories according to their shape and where they cut across the body:

Deep V neckline

A halter top

A woman in a tube top

T J Hart in a plunging neckline dress

The shape of a necklines can be modified in many ways, e.g., by adding a collar or scarf, overlaying it with a gauzy material or decorating the edges with scallops, picots or ruffles. The neckline can be a sharp edge of fabric or a more gentle cowl, and can also be accentuated by pattern(s) in the fabric itself.

Ruffs were popular in the Elizabethan era.

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Wikipedia