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Patternmaking: Slashing & Opening


When I first started working in technical design and was still training, I had the opportunity to work with many technical designers and pattern makers. Handling jackets, woven dresses, knit dresses, woven skirts, knit skirts, sweaters, loungewear, and intimates, I saw many fit issues. Rises, sleeves, armholes, I dealt with it all. It was frustrating at times to bounce from one tech and one category to another but it gave me insight into a hidden secret of the industry – that there are many ways to correct a one fit, pattern, or sewing issue and that one correction is not superior to the other. It’s all about if and how you back up your reasoning – it’s all about the perspective on the problem. There were many times when one tech told me to eliminate drag lines by doing A and when I went to another tech for approval, that tech would tell me to eliminate drag lines by doing B. Each tech correctly reasoned through their perspective on the issue so I considered each right.

The relationship of slashing and opening is another perspective issue.

The first month I joined the tech team, I was working on ‘fit comments’ for a jacket and we (I was an assistant at the time) were trying to reduce the bust dart intake so that the jacket would be less missy and more boyish (this was the request of the designers). As I verbally worked through my solution with my manager, she said to me, “But the amount you close isn’t always the amount that will open.” What?!?! She whipped out a post-it to show me but she had to scurry off to a meeting before we could get more in depth. I thought about what she said all afternoon – “So if I close a dart that is 3 1/4″, the amount that is opened won’t be 3 1/4″? Why?”

This techie tidbit has always stayed in my mind – I’ve never really gotten it. Every time I slash and open or slash and close, I measure the amount closed and the amount opened to see if I can prove my manager right. Nope. Every time, the amount closed and opened equal.

Have you ever heard of this? I would love to hear your perspective…


  1. Reply


    Well it depends on what you mean by “the amount you close isn’t always the amount you open.” If you close at an angle of 30 degrees, you will open elsewhere at an angle of 30 degrees, however if you’re talking in terms of inches that it opens, then no, it will not necessarily be the same. You can prove this very easily using a trigonometry operation. Draw a straight line from the point from where the dart starts, bisecting the angle, all the way to the end, and we we call the length of this line “x”. At a right angle to the line you just drew, at the end of it, draw another line which represents half the width of the dart (ie, the 3.5″ you’re referring to, but split in half), we will call this length “y”. Now, tan (angle) = y/x, and assuming an angle of 30 degrees in this instance, the ratio y/x will equal 0.58, meaning that if you use the slash and spread technique for this angle, the width of your dart will have to be 0.58 x length of your dart. The reason that you haven’t observed differing numbers using your slash and spread technique is because the bust point is nearly centered on the bodice, so the length will not change significantly, unless there’s something I’m missing here. This concept is often referred to as “similar triangles,” so here’s a link that explains it better than I do http://www.mathsisfun.com/geometry/triangles-similar.html

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      HANNAH! This makes perfect sense! My sophomore year of geometry is coming back to me! This question has been unanswered for years and I cannot thank you enough for finally answering it!

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        Oh good! I was worried it wouldn’t make any sense at all; describing and reading geometry clearly in paragraph form becomes complicated very quickly.

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          yes it does but once I saw the pictures from the post/link you provided, the light bulb went on and I got it. Thanks!

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          also, what is your email address? I’ve tried to find it on your Tumblr page before (I’ve been following you for a little bit) but I can’t find it!

          • hannah

            Oh, I’ll have to send that to you, sorry I missed this earlier!

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      I found this blog looking for patternmaking instructions and have been browsing the archives. When I read this I was excited that I immediately understood the answer to the question and would be able to explain it (I’m a math teacher and teach trigonometry among other things). But it looks like I’m too late! Your explanation is excellent!

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    I second Hannah’s answer to this. The angle you are closing will be the angle you are opening – not the amount in inches.

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    I agree with Mary and Hannah. To put it another way, the amount (in inches) you are opening in the new dart, will only be the same as the amount you closed in the old dart, if the legs of both darts are the same length.

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    Lisa Blank

    I see others have already given you the answer. Perfect!

    I was thinking of a practical example when I came here to comment. Try playing with a back shoulder dart. That’s a dart that is relatively short and narrow. Move that dart around and you’ll see how it differs compared to moving a bust dart.

  5. Reply


    Ahhhh! I see! This is a fun little revelation to me as well – I think I’ve observed the fact that “the amount closed is not the amount opened” but I never really understood the principle. It makes sense that it has to do with angle of the opening, NOT the inches!
    Learn something new everyday…. Thanks Maddie!

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    Geometry lesson! I love it.

  7. Reply


    This is totally new to me. In fact, none of my professors have ever brought it up, but what Hannah explains makes total sense!

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    Glad the mystery got solved, this is a good trick to know! Thanks for the info!!

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    Becky Stets

    Just work with spandex like me everyday and you won’t have to worry about darts!

    I feel like I need to go back and look at my pattern books now, I haven’t work with darts for far to long it looks like.

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    What an interesting blog post and awesome answer in the comments. šŸ™‚ Oh patternmaking. You are so scary yet fascinating, lol :p

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    It’s wonderful to find pattern making posts written by someone with real, recent experience at a company whose clothes I like. THANK YOU.

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