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.


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

  1. rachsews says:

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

  2. Gail says:

    Genius! Must try this some time!

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

  4. Geni M says:

    Wonderful tip!

  5. Katy Patzel says:

    This is SO clever!

  6. Jenny says:

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

  7. Janet says:

    wow, that is so nice! thanks.

  8. 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.

  9. Rose says:

    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!

    1. Jenny says:

      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!

  10. Nancy says:

    Great finishing technique! Thanks!

  11. Chrisagriff2 says:

    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?

    1. Jenny says:

      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.

    2. 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…

      1. Lois McAdoo says:

        Jenny, thank you for the very clear video. I’m going to experiment with a thick fashion fabric that wants to ravel, and I’m reluctant to trim my fashion fabric/lining seam to 1/8”. I suppose will need to add 3/8” more to each lining seam allowance, to wrap around the extra 1/8” of width and include 1/8” for turn of cloth. The garment seam allowance will stay at 5/8”. If you have any suggestions, please let me know! Thanks again.

  12. Ree Made says:

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

  13. blondie says:

    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!

  14. Awesome! Thanks so much for this. I’m sewing a cardigan from felted wool and this should work so well if I flatline with bemberg for comfort.

  15. Wendy says:

    Love your site do you have any thoughts or instructions on how to flatlining a cape, what seams finishes would you use. The pattern I am using is sim 8739

  16. nadeen-greetings from Tokyo, Japan says:

    I think this is absolutely an amazing tutorial… the results take my sewing to new level! thank you… thank you! xox

  17. Cynthia Lynn says:

    Excellent tutorial on flatlining. Thank you!

Let me know what you think!