Some relationships are toxic, some relationships are paid for, and some relationships are an embarrassment that should only live in the past. Fortunately, the relationship between across back, across shoulders and across front isn’t any of these. It’s a relationship that will stand the test of time, for better or for worst.
Across back, across shoulders and across chest are points/landmarks on a pattern that are used to draft and gauge armhole shapes. Across shoulders is shoulder tip to shoulder tip, and across front and across back for most sizes are approximately 5″ below high point of shoulder (HPS), which is where the shoulder seam meets the neckline. The difference between the across shoulder and across front/across back tells us whether the armhole shape will be good or god awful. Too big of a difference means that the armhole is too scooped and too little of a difference means that the armhole is too straight.
For a woven, across front should be approximately 1 ½” less than across shoulders and across back should be ½” less than across shoulders. So, if my across shoulders is 13 ½”, then my across front should be 12” and my across back should be 13” (these are actual measurements from my block – I’m not making it up). Because the fabric stretches, across front on a knit can be 2”, maybe slightly more, less than across shoulders and across back can be 1”, maybe slightly more, less than across shoulders. I use these measurements loosely as everyone’s body is different. This should not be an absolute rule but a guideline. When I evaluate a pattern, I use this formula to analyze the armhole shape and when I draft an armhole, I’ll use this to guide how I draw the curve. It’s just another trick of the trade. Enjoy!