March 24, 2014

Flatlining Tutorial: Finish underlining and seams in one (a.k.a. “flatlining”)

I discovered a great technique for finishing seams and underlining in one – it ends up looking like a cross between a French Seam and a Hong Kong finish.  It’s a perfect solution for silk or other fabrics prone to fraying, like my recent silk faille Simplicity 2343. I’m not sure from where it originated, but there’s a text tutorial on Pattern Review and a few bloggers have written about it before. However, I struggled to understand the instructions without some photos, so I thought I would put together this little tutorial for you! Hope you find it useful. The instructions below assume that your patterns has a 5/8 inch seam allowance.
How to do flatlining by Cashmerette

1. Cut your underlining 1/2 inch wider than the fashion fabric on each vertical seam (i.e. the whole piece will be 1 inch wider).


2. Sew the vertical edges, right sides together, using a 1/4 inch seam. Because the underlining is bigger, it won’t sit flush to the fashion fabric.


3. Cut the seams allowances down to 1/8 inch. I highly recommend the tip Amy learned at a sewing expo recently – use your serger without needles in to quickly and cleanly trim the seams.
4. Turn inside out (i.e. wrong sides together). Press the underlining away from the fashion fabric, and then fold the underlining around the edge of the fashion fabric to the back side. Note that the fashion fabric *doesn’t* fold, but rather, the underlining wraps around it. You can stitch in the ditch along the fashion/underlining seam – I didn’t do this for my silk skirt and I haven’t had any problems, but depending on the type of fabric you’re using you may want to do it.


5. Now treat the fashion fabric and underlining as one piece! When you sew it together, you’ll have lovely 1/8 inch wrapped edges. Remember that you trimmed 1/8 inch off the fashion fabric, so sew up the vertical seams with a 1/2 inch seam allowance.



Et voila! Give it a try and let me know if it works for you.
Also, the giveaway has closed! ChriChri congrats, you won the Liberty buttons and bias tape! Please contact me so I can send it to you.




18 Responses to Flatlining Tutorial: Finish underlining and seams in one (a.k.a. “flatlining”)

  1. rachsews March 24, 2014 at 3:19 pm #

    Great tutorial, thanks! I now want to underline something to ty it out!

  2. Fabric Tragic March 24, 2014 at 4:37 pm #


  3. Gail March 24, 2014 at 5:48 pm #

    Genius! Must try this some time!

  4. Sew little time March 25, 2014 at 4:44 am #

    what a great idea! really clear turorial thanks! i am planning a lace dress and so i think it will come in very handy!

  5. Geni M March 25, 2014 at 6:32 am #

    Wonderful tip!

  6. Katy Patzel March 25, 2014 at 7:03 am #

    This is SO clever!

  7. Jenny March 25, 2014 at 9:30 am #

    Thanks everyone! I hope it works for you – let me know if I got any instructions wrong 🙂

  8. Janet March 25, 2014 at 5:01 pm #

    wow, that is so nice! thanks.

  9. opportunityknits March 27, 2014 at 11:03 pm #

    Thanks for such a clear tutorial. You made it so simple, and I will definitely use it the next time I sew with a sheer fabric.

  10. Rose March 30, 2014 at 9:08 pm #

    Thanks so much for taking the time to post this! I’d heard of this technique, and seen a photo of the results, but honestly couldn’t quite work out how to do it — you make it so straightforward!

    • Jenny March 30, 2014 at 9:11 pm #

      You’re welcome! I’d read about it too but could never get my head around it without photo instruction 🙂 Hope you are able to use it!

  11. Nancy April 12, 2014 at 9:35 pm #

    Great finishing technique! Thanks!

  12. Chrisagriff2 May 28, 2014 at 6:02 pm #

    If I didn’t trim to 1/8, but left that, I’m assuming that would be an issue, but how so?
    Would I need to add more to my underlining?

    • Jenny May 28, 2014 at 6:16 pm #

      You could do that, but you’d just have to bear it in mind when you’re doing the final seam, to make sure you’re using the right seam allowance.

    • griffinfoodandsewing May 28, 2014 at 9:12 pm #

      Would it be 3/8 instead? I’m trying to check my math, which isn’t my top skill… Or really a skill I have…

  13. Ree Made July 19, 2014 at 2:27 am #

    Wow! How clever!!! Going to try this pronto. Pass me some silk!

  14. blondie July 18, 2017 at 4:58 pm #

    Thank you so much! I had tried to understand this from the one on pattern review, but couldn’t get it until I saw your very good explanatory drawings…. makes it so much clearer, thanks!


  1. Mistral Winds Kimono | - September 2, 2015

    […] because I’ve already spilled coffee on it.) I underlined it with white cotton lawn, using the Flatlining tutorial outlined on Cashmerette. This finished the edges beautifully, and the cotton lawn made the […]

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