Dividing and Truing Your Block

Recommended resources

The freely available resources this lesson will direct you to.

Welcome back! In our previous lesson, you skillfully manipulated the front bodice block, rotating both the side-bust and waist darts into a single, elegant armhole princess seam line. This was a significant step in translating a standard block into your custom design.

Now, we will finalize this pattern transformation. This lesson focuses on two critical actions: first, we will physically separate the front and back blocks into their four distinct panels. Second, we will perform the essential pattern drafting step known as "truing," where we check and refine the new seam lines to ensure they will sew together perfectly. This process is much like the final validation phase in product design, where you ensure all individual components integrate flawlessly to create a polished and functional whole.

By the end of this lesson, you will have a complete, "production-ready" set of four pattern pieces for your V-neck top.

1. Separating the Blocks into Panels

You currently have a front and a back bodice block, each with a new princess seam line drawn on it and, in the case of the front, a large wedge opened up by the dart rotation. It's time to create the final panels.

You will need your front and back bodice blocks, paper scissors, and some pattern paper and tape on hand for the next steps.

Separate the Front Panels: Take your manipulated front block. Carefully cut along the entire princess seam line you created, starting from the waistline, continuing up through the bust apex, and ending at the armhole. You will now have two separate pieces: a Center Front panel and a Side Front panel.
Separate the Back Panels: Now take your back block, where you drafted the armhole princess seam two lessons ago. Perform the same action: cut along the princess seam line from the waist to the armhole. This will create your Center Back and Side Back panels.

You should now have four distinct pattern pieces, which are the core components of your top. They will look similar to the sketch below.

Your four new pattern pieces: Center Front, Side Front, Center Back, and Side Back. The curved princess seams now contain all the shaping previously held in the darts.

2. The Art of Truing: A Patternmaker's Quality Check

Before we can use these new patterns, we must "true" them. Truing is the process of checking and perfecting seam lines to ensure:

Length: Seams that will be sewn together are the exact same length.
Smoothness: Lines that cross over a seam (like the hemline or armhole) are smooth and continuous, without any points or dips.
Right Angles: Lines that will be placed on a fold (like the center front and center back) meet the fold line at a perfect 90-degree angle.

This step is non-negotiable for a professional finish. It prevents puckering, mismatched seams, and an unbalanced garment.

How to : Check and true sewing patterns — In the Folds

The blog In the Folds provides an excellent and comprehensive guide to the principles of truing. This reading will establish the "why" and "how" behind the steps we are about to take.

Please read the following sections: Start with the introduction, from the beginning, to understand the two primary methods for checking seams. Next, read the section "Truing" PATTERNS. Pay close attention to the diagrams showing how to correct unbalanced armhole and hem lines. Finally, read the section Check that patterns are square. This is crucial for your Center Front and Center Back pieces.

3. Truing Your Princess Seams: A Step-by-Step Guide

Now, let's apply these principles to your new princess seam panels.

Step 1: "Walk" the Seams to Check Length

The most accurate way to check curved seams is to "walk" them. This means pivoting one pattern piece against the other along the seam line, simulating how they will be stitched.

This technique can be tricky to describe in text, so watch the following video for a clear demonstration. The presenter is truing a princess seam just like yours.

Pattern drafting for 3 different dresses with princess seams

This clip from Needle Eye Patterns provides a perfect visual of walking a seam, identifying a length discrepancy, and correcting it.

Watch the segment from this timestamp. Observe how she aligns the notches, pivots the top pattern piece along the curve of the bottom one, and marks the excess at the end to be trimmed.

Perform this "walking" technique on both your front and back princess seams:

Walk the Side Front against the Center Front.
Walk the Side Back against the Center Back.

Do the lengths match perfectly? If they don't, move to the next step. If they do, you can proceed to Step 3.

Step 2: Correcting Length Discrepancies

It's very common for small differences to appear after dart manipulation. If you found a discrepancy, you'll need to adjust the lengths to match.

How to : Check and true sewing patterns — In the Folds

Let's return to the In the Folds article to see the professional method for fixing mismatched seam lengths.

Read the section titled If seams are different lengths. The key is to split the difference, adding a small amount to the shorter piece and trimming the same amount from the longer one, rather than changing just one piece.

Apply this method to your seams until they walk together perfectly.

Step 3: Smooth the Apex and True Transitions

After rotating the darts, the point at the bust apex on your pattern can become quite sharp. This can create a pointy, unflattering shape when sewn.

Smooth the Bust Curve: Using a French curve, slightly round off the sharp point at the bust apex on both the Center Front and Side Front pieces. You are not changing the length, just softening the corner into a gentle curve. The Closet Core Patterns blog article, "How to Convert Darts to Princess Seams," highlights this in its instructions: "Smooth any sharp corners at the apex."
True the Hemline: Place your Center Front and Side Front pieces together, aligning them at the waist. Check if the hemline forms a single, smooth line across both pieces. Do the same for the back panels. If there is a "step" or a sharp corner, use your ruler or curve to redraw the line smoothly across the seam.

Step 4: Square the Center Fold Lines

Your Center Front and Center Back panels will be cut on the fold of the fabric. To ensure they open up into a symmetrical piece without a V-shape or "beak" at the neckline or hem, the pattern must meet the fold line at a 90-degree angle.

Using a set square or gridded ruler, check that:

The V-neckline on your Center Front piece meets the center front line at a 90-degree angle for a short distance (about 1cm).
The hemline on your Center Front piece meets the center front line at a 90-degree angle.
The back neckline and hemline on your Center Back piece meet the center back line at a 90-degree angle.

Adjust these lines as needed. This was well-illustrated in the In the Folds article you read earlier.

Key Takeaways

Congratulations! You have successfully completed the pattern drafting phase for your top.

You have transformed your basic blocks into four distinct panels: Center Front, Side Front, Center Back, and Side Back.
You have learned the critical quality-assurance process of "truing" a pattern.
You know how to "walk" curved seams to check their length, smooth curves at the apex and across seams, and square up lines that will be placed on the fold.

Next Steps

Your paper pattern is now complete, finalized, and ready for fabric. In our next lesson, we will move from the 2D world of paper to the 3D world of fabric. You will learn how to cut and assemble a muslin prototype of your top. This will allow you to test the fit and make any necessary adjustments before cutting into your final, precious material.