Some relationships are toxic, some relationships are paid for, and some relationships are an embarrassment that should only live in the past. Fortunately, this relationship – 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 POMs (tech jargon for points of measure) that are used to draft and gauge armhole shapes. Across shoulders is easy to find and measure – shoulder tip to shoulder tip. Across front and across back, however, are a little more difficult to locate and measure. Where is mid-armhole? Is it found by measuring along the curve of the armhole or straight from shoulder point to AH/SS? Because of this, there is a “standard” placement for across front and across back. For a size 6, across front and across back are 5” below HPS (another tech jargon term for high point of shoulder). Since these points grades 1/8”, subtract or add that amount (1/8”) to locate across front and across back placement for your size.
At my last job (as of a week ago, I worked in the technical design department for this fashion company), the first thing that was done to a fit sample when it arrived in-house was to measure it. In addition to critical POMs such as body length, armhole length, and bottom sweep, across front, across shoulders, and across back were measured. The difference between these POMs would tell us whether the armhole shape would be good or god awful before we even put the garment on the form. Too big of a difference meant that the armhole would be too scooped and if it was a sleeveless shirt, it would gape. Too little of a difference meant that the armhole would not be scooped enough.
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 knits stretch, the difference between these three POMs can be greater. For a knit, across front should be approximately 2” less than across shoulders and across back should be approximately 1” less than across shoulders.
Everyone’s body curves different, yes, so this should not be used as an absolute rule but as a guideline. When I evaluate a pattern, I use this formula as a guide for analyzing its armhole shape and when I draft an armhole, I’ll use this to guide how I draw the curved shape. It’s just another trick of the trade. Enjoy!