Full Upper Back Adjuster
Pulling and tightness across the upper back between the shoulder blades means you need more width there. Get the adjustment for your pattern type.
The total additional width needed across the upper back. If the back is pulling horizontally between the shoulder blades, estimate the amount — typically ¼"–¾" (0.5–2 cm). For a more precise measurement, pinch out the excess at each side seam.
Identifying a full upper back
A full upper back means the width across the back — from shoulder blade to shoulder blade — is greater than what the pattern assumes. On a finished garment, this shows as horizontal pulling lines, tightness across the upper back, or the sleeve being pulled backward.
To estimate the adjustment amount: measure the horizontal pulling lines with the garment on, or compare your back width measurement (taken across the fullest part of the upper back) to the pattern's back width measurement at the same point.
Don't confuse this with a high round back (excess fabric / a fold, not tightness) or shoulder width issues (which affect the shoulder seam position, not the back panel width).
CB seam vs. cut on fold
If the back has a center back seam, adding width there is the simplest fix — no slashing required. Both sides of the seam get added to, giving the full extra width when sewn together.
If the back is cut on fold (no CB seam), the adjustment must be made by slashing and spreading the single back piece. The slash runs parallel to the grain, roughly a third of the way from CB to the armhole — this spreads the width evenly without distorting the CB fold or the armhole seam.