What is "quality" clothing, anyway? Slow Fashioned has some answers
I've enjoyed reading Slow Fashioned articles since I discovered them a few months ago. Recently posted is a great article about something I think about a lot; quality. In How to spot quality in clothing: they explain several details and finishes to look for when purchasing apparel. This article covers key factors in garments being well-made, though I would add to this list the quality of the fabric itself, not just the labor and construction techniques that go into making the clothes.
My personal preferences for defining quality includes the soft feel of the fabric, and that it lasts many washings without pilling, changing size, or developing holes, which is unfortunately hard to determine before you buy the garment. I also like fabric that is thick enough to not require another layer of underneath it. Remember when most shirts were thick enough that you didn't need to wear a tank top under it so your bra doesn't show? Paying less for a shirt isn't better economically if I have to buy two of them. I'd rather pay a little more and not have to wear two shirts at once to prevent it from being transparent, but it seems more and more that that option isn't available.
Another common complaint is about polyester/synthetic fabrics, which rarely work well for me. I nearly always choose natural fibers so that they breathe and are more comfortable to wear throughout the day without getting clammy and holding the odor of perspiration. The downside of 100% natural fibers is that they tend to wrinkle and crease more than synthetic fabrics, but I would rather have a few wrinkles than fabric that gets clammy and smells bad, or has unknown chemical treatments to prevent it from wrinkling. I've read that formaldehyde is used as an anti-wrinkle fabric finish - something I don't want next to my skin!
It is interesting to note what a SF-Bay Area apparel professional recently pointed out: for offshore garment production the highest cost is the fabric since labor costs are so low. Therefore they compete on having the lowest cost fabric, which can translate to lower quality fabrics. Domestic garment production has higher labor costs, so keeping construction costs to a minimum is most important, and the fabric isn't driving the total cost of the garment to the same extent. That means in relative terms the focus on controlling costs is different, but of course fabric cost does matter in both scenarios.
What do you think adds to the quality of clothing?

