Lille Racerback Keyhole Hack

Posted by Emily Stevens on

Hi everyone! I’m Emily Stevens, and I am excited to share my Keyhole Back Hack for the Lille Racerback tank. I think this is a great way to show off your cute Power Sports Bras!

I will walk you through modifying the back of the Lille to create your new pattern, a few (quick) extra assembly steps, and tips to calculate and attach the arm binding. For reference, I do a size small and grade out through the waist and hips, and I did not remove any length from the pattern.  Keep in mind that less is more with this hack (I’ll be reminding you of this at key points), so I would not recommend widening the keyhole curves until you have tried this on. Once you’ve created your back pattern piece, this is a quick sew for a fun look!



To create your keyhole back pattern, you should start by either tracing out a “full back” version of your Lille back, or taping two copies of the back together. The “FOLD” line will become a line down the center back.



Next up, we are going to create a notch to overlap at the upper neck. My pictures are going to show this in marker so it’s easier for you to see, but the math teacher in me strongly recommends using a pencil until you think you’ve got your pattern done just right!

  1. Starting at the top of the fold line (now the center back), measure to the right ¾” and make a dot. You can choose to do more or less, but I found that this was easier to handle later on!
  2. Measure straight down from that dot ½” (parallel to the fold line) and make another dot.
  3. Draw a line straight down from the top dot to the bottom one, and then a line straight over from the bottom dot to the fold line.

The next steps will create the small keyhole at the top!

  1. Starting at the bottom of your notch, measure straight down the fold line. I am shorter and working on a size small, so opted for 2”. If you add length to the Lille, you might consider going a bit further down! Draw a dot at this point.
  2. Draw a gentle curve connecting your dot and the bottom of the notch, working on the left side of the fold line.
  3. My curve only went ¼” out at the furthest point. Remember, less is more with this one!

These next dots are going to help us draw our lower keyhole.

  1. Draw a (light) line connecting the bottom corners of the two armscyes. This will be perpendicular to your fold line/center back. Draw a dot where this line and the fold line/center back meet.
  2. Starting at this dot, measure straight down the fold line and make another dot. This will be the bottom of your keyhole opening. I went with 4 ½”, but you can do more if you want a more daring back (or add length to the Lille).
  3. The third dot I draw is on the left armscyce. The curve is very gradual at the top, then starts to grow more drastic as it curves around. Put your dot on the armscyce just before the curve increases.

Now for some fun curves!

  1. Draw a gentle curve connecting the dot on the armscyce to the first dot we drew in the last step (the center dot that is level with the bottom of the armscyce). You want this curve to very gently cup down.
  2. Continue from the center dot down to the one that is 4 ½” below, curving *very slightly* on the right hand side. I went no more than ⅛” away from the center with this curve. Again, less is really more with this hack!

  1. From this bottom dot, draw a short line directly to the left, extending ⅜”.
  2. From the end of that segment, draw a line all the way to the bottom of the pattern, parallel to the fold line/center back. Instead of cutting our pattern on the fold, we will be cutting two mirror, so need to add a seam allowance for the bottom center!

The last thing to do is to finish the cross strap at the top. I decided to scoop out the arm opening a tad, but you can likely skip this step with the upcoming update for the Lille!

  1. The dots on the picture show how far from the armscyce and approximate location I am going to scoop. On the right hand side, I did 1” below where the curve starts to level out, ½” below a bit further on, and ¼” just before the side seam.
  2. Draw a gentle curve connecting through from the bottom of the upper keyhole, through the three dots, to the bottom of the right-hand armscyce.
  3. If not adjusting the armscyce: connect your curve through to the original armscyce where shown in red below - this will be where your armscyce curve becomes less pronounced.

I would recommend tracing out your back pattern onto new paper and cutting out from there! Then you have your original if you need to make any adjustments in the future. Don't forget to trace the stretch and grain lines.

It should look like this:

Cut out your pieces! You will want to cut two mirror images of your back, and then the original Lille front and neck binding for the neckline you select. We will be measuring later on for the arm binding, so skip those pieces from the original pattern.

Lay your back pieces right side together, lining up our added seam allowance. Stitch from the bottom of the lower keyhole to the bottom of the back using ⅜” seam allowance.


  1. Lay your back piece flat and right side up. The straps will cross over each other and the notches should overlap.  
  2. Place your front piece right sides together with the back, and pin together at the shoulders and side seams. Also pin the overlapped notches.
  3. Stitch the shoulders and side seams using at ⅜” seam allowance. Baste the overlapped notches in place (I do a row of stitches at the top and bottom of the notches to hold them down well!)

PAUSE HERE! This will be a good time to try on your top and make sure you like the size of your openings. If you want either keyhole to be wider, or the lower one to extend further down, you can trim off excess fabric. Make sure you reflect those changes back on your pattern!

Now we need to calculate our arm/keyhole binding.

  • I measure starting at the center of the overlapped notches, and follow around the armscyce curve until I get to the center back seam. This will be half of your arm/keyhole raw edge.
  • Take this number and multiply it by .9, then add in .75 (for two seam allowances).
    For example, I measured 36”, so did:  36 x 0.9 + 0.75 = 33.15
  • Cut two bands using your measurement from above by 1.5” - stretch goes along the long length! (I cut two at 33” by 1.5”).

    1. Sew the two short ends of your arm binding together with a ⅜” seam allowance.
    2. Attach right side of the binding to the wrong side of your bodice, stretching slightly.
      I find it easiest to start by lining up one of my seams on the overlapped notches, and stretch slightly as I follow around the curve again. The second seam should line up with the center back seam, and then continue following around (stretching the binding slightly) back to the notches.



    Follow the directions in the pattern for attaching and finishing your binding, and then do the same for your neckline. (I like the cleaner look I get at the center top of the back when I do the arm binding first and then my neck binding.)

    Hem the bottom of your tank per the directions, and you’re done!



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