Knit fabric is made of yarns or threads looped around each other. When the fabric is pulled, the loops pull against each other. The space in between creates space for the fabric to move and stretch.
Here are a couple of examples of fabric: - Rayon spandex - Cotton spandex jersey - Sweatshirt fleece
To figure out the stretch percentage, line up the cut edge of your fabric next to a ruler. Hold the fabric at the selvedge and at 4″ and stretch the fabric as wide as it goes. Then release it and see if the fabric returns to 4″.
Each needle comes in different sizes, which correspond with the weight of your fabric: smaller needles go with lighter-weight fabrics, and bigger needles go with heavier-weight fabrics.
The most accurate way of cutting out knit fabric is to use a cutting mat, pattern weights, and a rotary cutter. Rotary cutters make cutting curves and notches easier.
Different sewing machine threads can produce different results for your project. - All-purpose thread - Cotton thread - Stretch thread