Shank - The shank is the part of the needle that is inserted into the sewing machine. The shank is the heaviest part of the needle and is designed so to minimize needle movement by attaching it firmly to the needle bar.
Shaft - The shaft is the narrow portion of the needle that supports the functional parts of the needle. Needle sizes refer to the diameter of the shaft. (Example - A #90/14 needle refers to the diameter of the needle shaft in hundredths of a millimeter measured above the scarf. "90" is the European/Metric measurement with "14" being the American measurement.)
Groove - The groove protects the thread by hiding it as it passes through the fabric on its way to join with the bobbin thread. Some needles have exaggerated grooves to protect the thread when sewing on particularly dense fabric. A needle that is too fine for the size of thread used will result in inconsistent stitches and broken threads.
Eye - The eye of the needle is the hole through which the thread passes. As the size of the eye increases, the size of the shaft increases to support it.
Point - The point of the needle is a primary distinguishing feature in needles. Points can be sharp or ball, or a hybrid of both. The angle of the point can be slender or acute. The point can be centered or eccentric. All are designed for a specific purpose and all give the operator unique applications.
Scarf - The scarf is the cut away portion on the back of the needle just above the eye. This area accommodates the hook mechanism as it rotates past the needle to engage the thread loop formed by the lifting needle. The shape and position of the scarf increases the consistency of stitching with various threads and fabrics.
We choose the needle based on the thread size.
Very fine needles (#70/10) for very fine threads such as #100, 60 wt. or monofilament.
Fine needles (#80/12) for fine threads (50 wt.)
Medium needles (#90/14) for medium threads (40 wt.).
Large needles (#100/16) for heavier threads (30 wt. and heavier).