First of all, general considerations are necessary to understand the right shape of a whippet neck. Neck must be considered concerning the breed. The conformation of the neck is subordinated to the shape, direction and dimensions. A neck could be: straight or arched.
Straight shape is useful for all the type of movements, while the swan neck, i.e. a neck is concave in the lower part, the head the head is carried high. Swan neck make the dog with full of elegance, but it is restricting for gate, because shift too on the back the center of gravity.
(translated from Sunto delle lezioni di Zoognostica e Cinognostica canina dei Dott.ri Giuseppe Solaro, Ignazio Barbieri, Walter Gorieri e Franco Bonetti)
Whippet FCI Standard N° 162 /18.10.2019
Long, muscular, elegantly arched.
Illustated Standard Whippet
Again, remember balance with nothing exaggerated. Fashion often seems to dictate that longer something is the better it is; this is not necessarily so. The neck should be long, muscolar and slightly arched to nable the dog to reach and grab a rabbit, which jinks from side to side in flight. The neck needs to be flexible enough for the dog to be able to make a 'side catch' if necessary. However, the neck should not be long and swan-like. To be balanced, the length of neck should be approximately the same as the length of the head. The elegant arch complements the flowing lines of the Whippet, and the neck should flow cleanly into the sloping shoulders. There should be absolutely no throatiness.
(from: Extended Breed Standard of Whippet - The National Whippet Council Australia)
The neck should be properly set with a smooth transition from the shoulders and topline with an elegant arch. The neck should balance the dog's proportions in general and not be overly narrow or long. The neck may also be too short. unfortunately a common fault which gives the dogs a less elegant appearance. This is more common today than a neck which is too long.
(from: illustrated breed standard, Svenska Whippetklubben 2014)