Sometimes (and even more often) we cannot have the original yarn, which is required to knit the design we've chosen. If you think it's a problem, think again: it's not a problem at all. The one and the only thing you should know is that you can knit everything using yarn substitutes. If the pattern says, you should use only recommended yarn it is not true. You can find hundreds of substitutes in your stash, on the internet or in the closest yarn shop. You can choose another brand. Merino wool is easily substituted with alpaca, cotton or simple wool as well as all kinds of blends. Linen substitutes with cotton or rayon, mohair with angora, cashmere with merino extrafine, camel wool or baby alpaca and vice versa. There are two more things you should know. Choose among the same yarn weight and after the yarn was chosen try to match the given gauge experimenting with different sizes of needles. Sometimes it doesn't work and mismatching the gauge even by half of a stitch can lead to the wrong size of a garment. It's handy to have a 1 or 2 ply yarns of the similar color in your stash to add to your main yarn and increase your yarn weight. Sometimes you can knit with the thin yarn held doubled and get an Aran, DK or Sports weight. If your yarn is slightly heavier, just change your needle size by one. The option of not having the original yarn shouldn't stop you from knitting your dream sweater. I have over a hundred finished projects on Ravelry. Only two of them had been knitted with the original yarn.