Just like across front, across shoulders, and across back, the relationship between front and back neck width is one that won’t fail. It doesn’t have to be paid for, it’s not an embarrassment of your past, and it will stand the test of time. Lovely how that works, ain’t it?
The rule states that back neck width is 1/2″ bigger/more than front neck width (measured straight – see sketch) and the reasoning has to do with horizontal balance. Since the shoulder seam sits slightly forward, extra width is needed to get around it. Also, the wider part of your neck is in the back; the skinny part is in the front. This rule gets tricky in that it only applies to close fitting necklines such as on a sloper or a collared shirt. Obviously, if the neckline is a wide, such as on a boatneck, extra width is not needed to get around the back neck; front and back neck width should be the same on such a neckline.
The pattern correction for this is easy but takes some critical thinking. Also, each person’s correction will differ because each person’s neck width differs. To correct the pattern, you will need to determine which neck width can be increased or decreased based on the patterns measurements. For example, the front neck width on my sloper/block measures 3″ and the back neck width on my sloper/block measures 3 1/4″ (remember that the pattern measurements are 1/2 the total measurement, so if my back neck width is only 1/4″ bigger than my front neck measurement on my pattern, the back neck width will be 1/2″ bigger than my front neck measurement on the sewn garment). If my front neck width measured 2 3/4″ (hypothetical number), I would increase it to be 3″ because my neck width is already small enough. Really, what you have to ask yourself is, “Can I make the neck smaller? Or can I make it bigger?” That all depends on the silhouette of the garment and how you want it to fit. After reducing or increasing front/back neck, ensure to also reduce/increase shoulder seam at armhole so that shoulder seam lengths equal.