Knit fabric falls into two categories: weft knitting and warp knitting. Weft knitting is a fabric knit in which the loops run back and forth, while warp knitting is a fabric knit in which the loops run up and down.
Knit fabrics are naturally stretchy. Certain fabrics have two-way stretch (stretching either horizontally or vertically) and others have four-way stretch (stretching horizontally and vertically). As a result, sewing patterns that use knits typically have something called 'negative ease'. Negative ease results in a garment dimension smaller than that of the wearer, forcing the stretchy knitted fabric to expand slightly when worn. Curve-hugging sweaters and warm knit caps illustrate negative ease. When the measurements of the garment and your body are exactly the same, you have no ease in the garment.