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Every yarn thickness has an optimum tension in other words every yarn because of its thickness works best on a particular size of needle. Too tight it is like cardboard and too loose it is baggy and when washed is more likely to lose its' shape. Getting the right pattern for your yarn is essential for happy results. So here we have given a bit of help by breaking patterns up in to approximate weights. Please note though that patterns are best used with their own yarn and swapping about doesn't guarantee success.
So always check your tension before embarking!