Altering Patterns for a Bias Grainline

I started working on a 40’s blouse over the weekend that calls for a closure-placket on a side seam.  Since I really dislike having a zipper closure on a blouse (if I can help it), I decided to alter my pattern to be on the bias.  Using a bias construction gives the fabric a bit more stretch so I can get in and out of it easier without needing a zipper.

While I’m sure many of you already know how to construct a garment on the bias, I thought this would be the perfect opportunity to show a quick and easy way to mark the bias grainlines on your pattern pieces.


  • Ruler
  • Pen/Sharpie
  • Scrap paper

I admit, I really didn’t need to include a supply list here.  But for the sake of being a tutorial, I had to include it.  :)

Step 1: Grab a piece of scrap paper, with straight edges

My scrap paper has a little V-marking on it for some reason.  heh

Step 2: Fold paper like so

Take one edge of the paper to meet the opposite edge, creating a sharp point and fold.

This folded piece of paper has the straight-vertical grainline on the left side as well as the newly folded 45 degree grainline, or bias-grain.

Below is my pattern piece that I’m altering from the straight-grain to be on the bias.  The original grainline marking is the vertical line in the center.

My tracing wheel and scissors are on my pattern piece acting as pattern weights.  :)

Step 3: Place the piece of paper underneath (or over) your pattern piece and align the straight grainline to match the pattern piece

 Step 4: Use your ruler to mark the new, 45 degree line for the bias

My pen is pointing to the line I just made using my ruler.  I actually made two 45 degree lines, one on either side of my ruler.

Here’s a detail shot of the original grainline and the new bias grainline.

Easy right?!

My bodice pattern pieces are to be cut on the fold, so below I’ve folded my fabric just like I did with my scrap paper.  You fold your straight edge to the selvage in order to create a 45 degree bias-fold, and the pattern pieces abut on the new fold line.

I’m all bias-ed up now.

I spent most of my weekend working on muslins (and also on my knitting) but was happy to be able to cut out some of my real fabric on Sunday.  Hopefully I can finish this blouse up soon, but it also means I’ll be doing some craziness: cutting & working with chiffon for the first time.  Wish me lots of luck!


In: Sewing Tutorials

Blogger for 6 years and counting, I am a passionate creator who loves to tinker.

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