Today there was a question in the Edgewater Avenue Group that made me realise there's a post I need to write ... about the difference between fabrics that stretch in two directions (4 way) verses those that stretch in one (2 way).
First let me say I hate the expressions 2 and 4 way stretch as they're so confusing ... one direction and two direction is the way it used to be and I still regard that as less confusing. I've written about this previously ...
"Let me start by saying there is a huge amount of confusion regarding the terms two way and four way. A long time ago it used to be called one way and two way stretch; one way meaning stretching perpendicular to the selvage only (typical of things like stretch velvet and stretchy denim) and two way meaning stretching parallel to the selvage as well. Somewhere along the way this became two way (the old one way) and four way (the old two way). If you come from the old system or during the changeover then two way could mean anything. So instead I say a fabric stretches in one direction or in two directions."
For the rest of this post I'm going to use the terms one direction and two direction to avoid confusion.
For the most part these days, it's very easy to get fabrics that stretch in both directions ... a long time ago it wasn't that easy. We're spoiled for choice now in fabric thickness, stretch percentage, rebound strength and blend ...not to mention all the textural and print options available ... we even have fabrics specifically designed for things like chlorine, UV, wicking/rapid drying, etc. These modern fabrics can be distorted in all manner of ways and handle it longer before rippling. But it wasn't always this way.
Long ago, stretch fabrics meant knitted wool ... and that wasn't really stretch but a loose enough knit that the fabric could distort ... think in terms of a zig zag being able to stretch ... the thread itself cannot stretch but the zig zag can pull straight ... this was how fabric stretched before. That zag zag stretching to become a straight line is the visual you need to understand that when you stretch a single direction stretch fabric, the perpendicular direction shrinks ... as you make the horizontal longer, the vertical shortens. With knitted fabrics in the past, before the addition of spandex fibers, that vertical reduction was huge ...you could lose as much as 50% from your vertical just by stretching 15-20% on the horizontal. Their greatest fault however was the complete inability to stretch on the vertical ... so much so that if you had no positive ease (they needed a good 6-8%) on the vertical then your garment wouldn't stretch on the horizontal to conform to the body at all! just think about that for a while! If you were at the small end of a size envelope you'd get gaping and if you were at the large end then you'd look like a squeezed ham. The size spacing had to be reduced to almost half of what it is today ... and getting custom made swim out of these fabrics was almost impossible back then.
Over time the technology improved. Fabrics with synthetic fibres and spandex, although still stretching in one direction, have drastically reduced the shrinking on the vertical, but it's still there. The main advantage of these is that you don't need as much positive ease on the vertical (around 3%), as you used to, meaning they can still conform reasonably well to the body with up to 12% horizontal negative ease and span the entire standard 4cm size envelope, just ... the US 5cm (2") envelope is really a touch wide but it's possible if you're an average shape (non-average shapes push that envelope a bit too far to the point that half sizes really are required). Making ready to wear garments that don't have shear ripples somewhere is almost impossible.
Two directional stretch is a whole different ball game. When you make a garment with, say 12% negative horizontal ease, you can put in on anyone within that size envelope and the vertical shrinkage, although there, will be minimal ... so it will conform to the body much better. The catch is that at the large end of the envelope you start to get vertical rebound limiting the horizontal stretch a bit earlier ... it'll still stretch and conform, but the likelihood of shear ripples increases slightly. This is the main consideration when it comes to not using darts around the bust ... with increased size and cup size, no darts usually means more ripples, especially in the higher rebound fabrics.