Oops, here ya go.
"Poor quality or the wrong type of thread can cause poor stitches on any
machine. Thread passes through the eye of the needle approximately
37 times in a “seesaw” action before it forms a single stitch. Poor quality
thread results in crooked and looped stitches, puckered seams, frayed
thread, and/or needle breakage. To achieve quality stitches, a sewing
machine needs three things: correct thread, correct size needle, and
properly adjusted thread tensions.
Several factors such as the fiber, twist, ply, finish, and size of thread
must be considered for use with today’s fabrics for fine stitches.
Staple – refers to the length of fibers used to twist together to form a
single ply. Domestic thread fiber lengths are usually 1˝” to 2˝” long
while European threads use 5˝” to 6˝” lengths.
Ply – number of strands twisted together to make a single thread.
Roll – the tendency of thread to roll to the right or left during stitching,
causing the stitch to appear slightly crooked. Poor quality threads are
more likely to roll.
Twist – the crimping of fibers which causes them to interlock firmly into
a single ply. Thread should not untwist during stitching. This will cause
skipped stitches, thread breakage, crooked stitches, and weak spots in
seams."