Single jersey is knitted on one set of needles. It has flat V-shaped stitches on the face and loops on the back, it is light and flexible, and it tends to curl at cut edges. Interlock is a double knit made on two sets of needles, in effect two 1×1 rib fabrics knitted together. It looks the same on both sides, is thicker and smoother, and curls far less. For the same yarn, interlock uses more yarn per square metre, which is why it is heavier and usually costs more.

What the wearer notices

Interlock feels smoother and fuller in the hand, is more opaque, and holds its shape well. It suits kidswear, polo-style tees and premium basics. Single jersey drapes more, feels lighter and cooler, and is the standard for most everyday tees. Interlock may stretch a little less widthways than single jersey of similar yarn.

Making up

Because interlock curls little, cutting and sewing are easier, and hems and necklines lie flatter. Single jersey edges curl, so handling and seam finishing need more care. Both can twist after washing if the yarn and finishing are not controlled; check spirality on washed garments and fabric bow or skew with a method such as ASTM D3882.

Testing both

Compare fabric weight with a tolerance using ASTM D3776, shrinkage after a named wash procedure such as ISO 6330, and pilling with a method such as ASTM D3512 on washed fabric. Results depend on yarn, cotton quality and finishing, not only on the knit type.

How to choose

Make tees in both fabrics and wear and wash them. Judge feel, opacity, shape after washing and the price difference per garment, not per kilogram of fabric. Interlock's higher weight means fewer metres per kilogram, so compare garment costs.

What to specify

Write the knit structure, yarn count and type, fibre content, weight with tolerance, width and shrinkage limits, and keep the approved washed sample as the bulk reference.

If you change from one knit to the other mid-season, re-approve fit, since the different stretch and weight can change how the same pattern sits on the body.