WHAT IS IT?
What if you want to add a button closure to a pattern that doesn’t call for buttons? It’s easy to modify zip-front styles by adding a button overlap to the pattern piece. The button overlap is an extension so that the layers can cross over each other, with a button sewn underneath each buttonhole.
WHEN DO YOU USE IT?
Anytime there are buttons and buttonholes, there has to be a button overlap added to each side of the opening. The only time that an overlap isn’t required is when the buttons close through button loops (page link) and the edges meet in the middle instead of overlapping. You may still want to have a button overlap with button loops on blouses or other garments where you want the fabric to cover the opening so no skin shows through.
HOW TO CALCULATE A BUTTON OVERLAP
The rule is that the overlap/extension should equal the width of the button being used. This ensures that the buttons sit nicely in the overlap and the overlap edge is proportional and pleasing to the eye.
From center front, measure the width of the button overlap from the top and from the bottom. Draw a line parallel to the center front line. This is your new overlap line! Connect the lines at the neckline and hem.
Source : The Sewtionary An A to Z Guide to 101 Sewing Techniques + Definitions
About the Author : Tasia ST. Germaine