It’s always important to swatch the yarn that you want to use for a project. Not only will swatching on different needle sizes tell you a lot about your yarn, it also lets you know ahead of time if your garment will fit as planned. If you don’t meet the numbers, the dimensions of your final project will be off. As an example, take a look at the Allegoro swatches pictured below. The top swatch is knitted at the gauge on the label and the lower swatch is knitted at the gauge used for the cardigan. Both swatches are knitted with the same number of stitches and worked for the same number of rows. Note the difference in width and height. If your gauge is very different from that in the pattern, not only will the bust measurement be significantly wider or narrower than what is intended in the pattern, but the shaping of the armholes, waist, and sleeves will also be off.
By the same token, if you substitute yarns, you might be able to get gaugeand dimensionsbut the look, feel, and drape of your final garment may not be what you want.