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1. Fabric Character

What Are Stretch Textiles Made of?

Elastomeric fabrics are a blend of elastane and one or more other fibres. Elastane is a copolymer fibre made from alternating segments of polyethylene glycol and polyurethane. It is not rubber but has similar properties, including memory, which is the ability to return to its original size or shape after being stretched or distorted.

Elastane is more durable than rubber, being significantly more resistant to chemicals like salt and chlorine, as well as detergents, alkaloids (sweat) and body lotions. Elastane, like rubber however, is damaged by heat and UV radiation (sunlight). As such, elastane blended fabrics should not be dry cleaned, machine dried or hot ironed, and should definitely be drip dried in the shade. Why then do we use such a fibre for swimwear which almost always is worn out in the sun? In the past, there has been no better stretch fabric suited to the task. Recently 100% polyester knits have begun come close to the performance of elastane blends although they tend to lose memory much faster so haven't yet gained widespread industry acceptance. Polyester knits are still likely to be the way of the future because they have better chemical resistance and can even tolerate the heat of dye sublimation printing. You can machine wash and dry them, and even iron them without risk of damage.

There are as many different blends and weights of elastane knits as there are non-stretch textiles, each performing differently. Generally there are some rules of thumb:

  • The higher the elastane content, the greater the stretch and the stronger the rebound tension.

  • The looser the knit the more stretch but the lower the rebound tension.

  • The heavier the fabric weight the greater the rebound tension, but the lesser the working stretch range.

  • Fabrics with natural fibres in the blend have a lower rebound tension and may collapse (lose their rebound tension) completely when wet.

Rebound is the force that causes the stretched fabric to return to its original size. When you take, for example, a 10cm long strip of stretch fabric and pull you will feel a force pulling back. The amount of force generated to return it to its original length will change depending on how long you stretch it. It may for example stretch easily out to 13cm then begin to get 'heavy' but continue to stretch out to 17.5cm. This would be called a 75% stretch as it can increase 75% over its original length. But if you think carefully about how the wearer will put on the garment and move in it you'll soon realize that it's only the first bit of 'soft' stretch that is functional. The objective, then, is to design your garment such it both fits correctly and can stretch comfortably within that initial stretch.

That said, there may be times you might wish to exploit the extra rebound tension to help stabilize your garments. Typically you'd do this for shoulder and back straps, or anywhere else that narrow strip sections are used in the design.

Two way stretch knits have one unforgiving downfall. They do not respond well to being stretched in the 45° cross grain (not that there is technically a cross grain in knitted fabrics). Stretch fabrics require the vertical and horizontal tensions to exceed any forces applied cross grain. You will know you have excessive cross grain tension when you come to fit the garment and you end up with a series of parallel angled ripples somewhere. There is another fit issue which results in non-parallel ripples, but we'll cover that later. Just be sure you can tell the difference between them!

Another consideration, if used, is the lining fabric. This component will add both extra fabric weight and rebound tension. If used in one section only (e.g.; power netting in a front body panel), the greater part of the rebound tension will be taken out of non-lined panels before the lined panel begins to stretch which may result in a poor fitting garment.

So what does all this mean? Much to the dismay of many designers and manufacturers, it means you may need to redraft your basic blocks each time your fabric characteristics change. For example, let's say you design your blocks for the average 180gsm, 80% nylon / 20% elastane, 75% two way stretch. This fabric is generally workable to 30% before getting heavy and works well with negative ease values up to 12%. If you were to change to a 220-240gsm fabric of the same content you'd find you were dealing with only 18% workable stretch meaning that while your block would still fit the body, the wearer would experience more trouble getting it on. Furthermore, the wearer would intuitively know the heavier fabric garment was 'too tight' and possibly suspect they required a larger size.

If you redraft your block every time your fabric characteristics change, no matter how small, you'll find your clients are much happier. If you don't believe smaller changes are practical or economical, at least understand the consequences.

2. The Principle Stretch Direction

Where Does The Negative Ease Go Exactly?

The objective of stretch fit pattern making for stretch wear is to create a pattern which will result in a garment of even (not necessarily equal) tension on the body in all directions. This is critical. If tensions are not even then the garment may ripple and move on the body. If you think of a one piece swimsuit as a tube that encircles the body there are two questions to consider. How long is the tube and what is its circumference? These are the two principle stretch directions.

To make a swimsuit ‘shrink’ onto the body at least one of these directions requires what is referred to as negative ease. Negative ease is where a body measurement is reduced by a certain amount in the pattern. It is the opposite principle to adding ease to a measurement for non-stretch pattern making. Generally, we reduce only the measurements that go around the body. There are two main reasons. Firstly, when a person twists and bends they do not significantly change any of their girth measurements, however they do change their body length. Thus adding limited or no negative ease in this direction adds a certain amount of softness to the garment when the wearer moves. If the garment is unable to stretch vertically, because it has already been reduced, it will move over the body or distort rather than change shape with it.

Secondly, though more a consideration for ready to wear manufacturing, it is best to allow for more variation in girth than height. You’ll fit more customers within each size if you set your principle stretch direction around the body. Try to consider each size as a range of measurements. For example, while we define a size 10 as an 84cm bust line, we would hopefully design a garment that would comfortably fit a bust of between 82 and 86cm, with similar allowance made for waist and hips.

The more horizontal negative ease you include in your block, the smaller the number of people your garments will fit within each size. Can you answer why softer surf style swimwear brands fit just about anyone, while firmer high fashion swimwear usually does not, yet still sits a lot better?

There is also a subtle third reason which is complex and often overlooked. Put simply, when you apply tension to a stretch fabric it will naturally tend to shrink in a right angled direction. The fabric is trying to relieve the tension you’ve applied to it. How much it counteracts depends on the fabric. Recent fabrics with higher elastane content do not counteract as much, some remarkably not at all. You will need to test your fabric and make allowance for this amount.

professor logo

The Professor says: Sometimes the fabric will decide the direction for you. A single direction stretch fabric might have a print running only one direction, for example. Another might stretch more in one direction than another. Usually the fabric’s greatest stretch direction should run around the body.

The table below illustrates the typical relationship between horizontal and vertical negative ease for 180gsm, 75% two-way stretch nylon elastane. Generally the more you reduce the horizontal, the less you need to reduce the vertical to apply the same even tension to the body. Which set is used depends on the fabric, designer and the garment’s intended purpose.

Understanding where the tension is being applied by the pattern is key to predicting where the garment will move to when on the body. If you tension varies across the body the garment will always try to move to the area of lowest tension. If it's higher in just one place then you'll get ripples appearing in the garment as the perpendicular tension collapses. An even tension across the entire garment is essential to a stable fit ... meaning traditional shaping techniques, such as darts, are even more important (contrary to popular belief).


Anyone designing swimwear needs to understand this principle almost second nature or they will forever be chasing themselves around in circles trying to work out why a particular garment keeps failing to properly fit or moves where it shouldn’t.

But how do you determine what total amount of negative ease is appropriate for any particular fabric the first time? You need to asses the fabric’s stretch and rebound properties. In the past many designers have simply wrapped a section of fabric around the bust until the tension ‘feels about right’, measured how much fabric was required at that tension and then compared it to the bust. For example, if it takes 74cm of fabric to comfortably wrap a bust of 84cm, you have a reduction of 74/84 x 100 = 88% or a negative ease of 12%.

These days manufacturers provide you with fabric performance statistics on request. They are required to perform national standards testing on all fabrics they sell. Which standard they use depends on the country and the measurement system used. This data should be able to tell you how much percentage reduction is required to create a certain tension. Some people still find this data difficult to interpret and resort to the old way of assessing a fabric, but I strongly recommend getting the manufacturer’s sales representative to explain how to interpret the system they’ve used as it will give you much more reliable and consistent results and save time in the long run.

For demonstration purposes this site will base all patterns on either 8% or 12% horizontal negative ease (it'll say in each case) and 0% vertical negative ease. These values suit the common 75% two way stretch spandex fabrics. Some people suggest that if you are making swimwear for the first time you might want to base your patterns on 8-9% horizontal negative ease and 3% vertical negative ease instead as this creates a more forgiving pattern ... it does, but then you'll need to change all your patterns again in the future once you've got the hang of it so I'd honestly go with no vertical negative ease.

Remember, every time you change your fabric the elastic modulus (how much force you need to stretch it) may change, so you may need to change your block, or the patterns you cut from them will not be the same. I have different blocks at different values of negative ease already prepared, so I choose which one I want based on the fabric and it's intended use.


red monokini

Photo by Chris Huzzard

3. Tension Line Science

How Does Tension Affect My Garment

Just about everything in stretch fit design comes down to tension lines. A tension line is used to illustrate any direction where the garment is being stretched. Let's consider a basic tank suit. 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 centre 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 tank suit and halter patterns. The curve is a throwback 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 direction of the tension lines.

tension line directions in stretch fabrics

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 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...

tension lines in stretch fabrics using anchor points

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 centre 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 that was your intention or not. The trick is to know how to increase or decrease the tension in each line appropriately to give the exact final shape you are after. 

Tensions are typically adjusted by lengthening or shortening sections. 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. But how do I know if something is distorted? How do I check the tensions?

Well that's sort of easy and not so easy at the same time. Long ago when I wasn't 100% certain about what tension was doing, I used to sew up my initial commercial samples in a white fabric with a 50mm grid drawn onto it so that when the sample was worn you could literally see where the fabric was stretching. Below are a couple of photos I found on Instagram ....

using print patterns to assess stretch amounts
using print patterns to assess stretch amounts

Photo found on Instagram @fabu.swimsuit (I do not own copyright); used for the purpose of illustration/education only.

I've used these images because of the gingham print ... the print shows exactly how and where the fabric is stretching under tension. What we're looking for as pattern designers is to create a garment where the tension is smooth and, if it changes, changes in a linear and gradual manner the gentle contour lines on a weather map. What we don't want to see is sudden angular change in the print/grid or exaggerated distortion or skewing of the lines. The print on the above swimsuit has really stretched both vertically and horizontally over the butt, but its still even and gradual ... this is how stretch fabrics contour around shapes on the body and it's just fine. We do need to watch for if the maximum size of one of those grid squares is far greater than the smallest (resting) square ... in the case above the largest is around 50% bigger than the smallest, which is fine locally, but if it had stretched this much over a larger area it would exceed the negative ease we put in the garment ... ie; it'd be too tight.

I've placed a whole stack of images with obvious linear patterns (gingham, stripes, houndstooth, etc) on my Pinterest ...what I want people to do is to look at the images again and again until they can see where the tension is in each ... you need to be able to visualize which area is tight and which is not, both locally and across the garment as a whole ... you need to be able to see it in a one piece such that when you cut through those tension lines to create a cut out style (as we mentioned at the top) you can see the tension springing back in your head.

While it may not sound very simple, with experience you will recognize 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.

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