The Science of Tension Lines

Just about everything in swimwear design comes down to the science of tension lines. Put simply, a tension line is any line that demonstrates where the garment is being pulled. Let’s consider a basic tanksuit. The actual bust line would be a tension line, as would the waistline and shoulder straps, for example. But consider for a moment if we created a cut out at the center back waist. The waist line would no longer be a tension line. And it’s the tension lines that hold the garment in place … and in a particular shape.

When you come to read the pattern making sections that follow one thing you’ll notice is that all my strap lines are straight rectangles. They are not curved like you see in so many commercial tanksuit and halter patterns. The curve is a throw back from non-stretch pattern making where the strap had to be curved so as to look straight when on the body. Many pattern makers haven’t understood the principle of tension: if you hold the ends of any shape of stretch fabric and pull, it will form the shortest distance between the two points, which is a straight line. Make your straps a straight line and they will still follow the bodies curves, only this time they’ll be wrinkle free and fit in a predictable manner.

Take a look at the illustrations below. Each shows the obvious location of major tension lines.

Something common to all tension lines are anchor points. An anchor point is typically found at the end of a tension line but may be anywhere in between. They even exist in circular tension lines that apparently have no end. It is a position that is somehow fixed in place on the body. They may be formed by hem elastics (tension lines in their own right) or by body parts or, most often, by intersection with other tension lines. Let’s consider things a little more simply …

The left illustration above shows a simple strap. The dotted line represents the tension line direction and the red dots represent the anchor points. If we add another strap to the first, this time pulling up to the right we get a tension map something like the center illustration. Add another strap and the map distorts again (right illustration). The important lesson to learn from this is that the garment will shuffle itself around until all the tensions are as even as possible, whether or not that was your intention. The trick is to know how to increase or decrease the tension in each line appropriately to give the shape you are after. If the leg hole of your garment, when on the body, doesn’t match your pattern design you can almost bet this is the reason

Tensions are typically adjusted by lengthening or shortening patterns. Lengthening an ‘off shoot’ tension line may help straighten up a ‘primary’ line. Lengthening the ‘primary’ line would cause it to be distorted more by the ‘off shoot’ line … which would also happen if you simply shortened the ‘off shoot’ line. So should you shorten or lengthen? What you need to do is consider what you want the overall garment tension to be and either shorten or lengthen the ‘primary’ line to achieve the correct shape and tension, and then lengthen or shorten the ‘off shoot’ line to do the same. And so on for all off shoots thereafter.

While it may not sound very simple, with experience you will recognise the lines, tensions and anchors automatically. As I create the patterns on this web site I will demonstrate how to straighten up pattern sections to line up with where tension lines will form on the body. We will even create a garment based on several intersecting lines, none of which are circular, that will seem to defy logic, yet sit perfectly flat.