This is one of my favourite topics: how to choose the right yarn for your project. I'll try to be brief! Bells recently wrote about knitting a cardigan in laceweight yarn and how disappointed she was when it quickly began looking worse for wear. Is it possible to knit a durable garment in laceweight, she wondered? My reply:

I think it is not so much the weight of yarn as the fibre content.. I’d be wary of knitting a garment in Malabrigo Lace, Sock AND Worsted, simply because of the way their merino yarns are constructed (maybe the sock less so but def the lace & the worsted). You’d want a relatively tightly spun lace and preferably in longer fibres than merino.

In other words, when you plan a project you need to take several things into consideration when you choose your yarn in order to avoid disappointment or a garment falling to bits. These should be your main considerations:

• Weight (lace, 4ply/sock, sport, double knitting etc)
• Construction (single ply, 2-plied, chained etc)
• Fibre (cotton, alpaca, merino, blue-face leicester, bamboo etc)
• Care/Maintenance (machine-washable? dry-clean? hand-wash? defuzzer?)
• Price

Two examples:

1. Baby Cardigan. The pattern will tell you a gauge which indicates which Weight you should use. Typically this would be a fine weight like 4ply or double knitting/light worsted. Then think about Fibre: you'll be wanting something soft, so merino would be a great choice (as would blue-face leicester). However, Care/Maintenance tells you that while that Malabrigo Worsted might be the softest thing on earth, it is hand-wash only which isn't a great choice for something that will get drooled on 24/7. It will also start to pill very quickly (it's basically a 2-ply disguised as a 1-ply - that's Construction) and new mums will not have the time to defuzz that baby cardigan before they leave the hospital. You should probably opt for something sturdier, but still soft. Price? That will determine whether you'll end up with a soft acrylic or a soft merino-blend. The choice is yours, but whatever you choose, you will end up with a great baby cardigan.
2. 4-ply Lace Cardigan. You have already been told by the pattern which Weight to use. You should also look at the recommended Fibre as it will tell you whether the cardigan is designed to be drapey (i.e. silk, alpaca, cotton) or to keep its shape (wool, linen to some degree). If you want to turn a 4-ply cotton cardigan into something you could wear during the winter, you would look for an alpaca 4-ply, for instance. If you will be using the cardigan a lot, you'd want a fairly sturdy yarn (high twist, maybe even cabled - again, use your knowledge of Construction) in a robust fibre (something like a Shetland or an Icelandic 4-ply - or a woolblend) whereas a statement piece could be done in a more delicate yarn (silk, pure merino). Here Care/Maintenance is also important as a soft, delicate yarn invariably will pill more than a rustic yarn. Price is naturally also important.

Designers can generally be trusted to make most of the decisions for you when they write their pattern, but sometimes even they make a poor decision and opt for a pretty yarn which may not suit your purposes (i.e. the pattern is written for delicate cashmere, but the actual garment is a practical cardigan). This is a pitfall for many very experienced knitters.

Notice how I haven't even spoken about colours or how the fibre feels against your skin? That is because I believe you need to have narrowed down your yarn shortlist before you start looking at those pretty, pretty colours or before you start stroking those soft hanks. Most knitters start with a colour before they start thinking about more technical stuff, but, really, would you buy a car just because you liked the colour? Be smart about your purchases, dear readers.

Finally, I know that you might not feel you're "smart enough" to walk into a LYS and find the right yarn based upon the criteria outlined above. This is why LYS employees are suddenly your best friends. They (should) know their yarns and be able to assist you in an honest, friendly and knowledgable way. Explain exactly what you are making, have your pattern's gauge/needle-size on hand, point out who you are making it for (this is more important than you might think) and what your price range is. It'll help them help you.

I don't know if I have been brief here, but I hope this has been helpful. Any comments or queries?