Sock Yarn v. Fingering weight yarn:: What are the differences? What are the similarities?

Recently when I was in a yarn shop with students, I was corrected by the owner.  We had been discussing yarns to make a lace cowl.  I had been suggesting a sock yarn could be paired with a lace weight to make the finished look desired as well as give the lace weight some heft since some were not used to working with something so fine.  What I was corrected on was the fact that the yarn she saw me pointing to was in fact fingering weight and not sock yarn.  I carried on with the lesson on yarns and the shop owner gave input where needed.  She after all should know her stock.

But besides the fact that we were both right (angle of perception has a great deal to do with what you see and think you see), it gave me pause to consider.  Am I shortchanging students in knowledge?  Better still, does anyone care?  I figure yes we do care.  Possibly this discussion is suitable for intermediate students though,

Sock yarn is yarn that is used to knit or crochet socks.  That is the base answer and while true is also untrue.  Sock yarn should be slightly stretchy.  It has very definite crimp in the fiber allowing for a lot of twist in the spin, thus allowing a lot of twist in the plying.  Sock yarn, if they are to be worn in shoes, is almost always fingering weight or a light fingering weight.  Much more and shoes won't fit over the foot and sock.  But is the goal for boot socks?  Then heavier weight yarn can be used.  Is the goal for bed socks or house socks?  Still heavier yarn can be used.

Fingering weight yarn on the other hand is any yarn that has 19 - 22 wraps per inch (how many times the yarn wraps around a ruler or pencil or any even object in an inch)  and knits up at a gauge of 7 - 8 stitches per inch.  It is fairly fine but not thread like.  The fiber may or may not have a lot of crimp.  The yarn may or may not be highly twisted.  The plying will match the twist.

On the surface, they seem to be one in the same.  But consider this.  All the sweaters, shawls, hats, scarves, you name it don't have near the friction  that a sock does inside a shoe, or even just being worn.  We don't walk on sweaters or shawls or hats.  This means that all that extra twist is more surface area for the yarn to wear evenly.   You won't get a wear hole in the sock nearly as fast as a yarn that doesn't have all that twist.  This is an important thing to consider. 

The shawl that is done in fingering weight yarn will usually have a lot of drape.  Fiber and gauge have a lot to do with this, but so does the amount of twist in the fiber as well as the amount of twist in the plying.  The shawl done in sock yarn but the same gauge and fiber won't drape as much.

So what is your preference?  Did you know there was a difference??