The shape of my face is oval or oblong. Without a standard for each, I'm not sure which it is. According to
this site which Wende referenced in the comments, I should be able to wear "any length" if I have an oval face, and if it is oblong, I should wear choker length necklaces because they reduce the perceived length of the face.
This site, tells me a choker length is 14 to 16" but that it is best I don't wear that length if I don't wish people to focus on "that part" of my body. Fortunately, I'm not so aged as that, yet.
Interestingly, I don't have anything in this length, nor in the shorter "collar" length (12-13" usually multi-strand) But it would be a good length for T-shirts wouldn't it? And collared shirts, with the collar open. I'd have to see. I know the 18" length I wear is a tad long for T-shirts. I do have an awful lot of 18" necklaces. Apparently this is the "princess" length and one's collar should be either higher or lower for this. (And on me, this is just about exactly where T-Shirt collars fall. That could explain why I don't wear them.)
The "matinee" length is 20" to 24." It's supposed to add elegance to work clothes and casual wear.
The rest of the necklaces fall into the "opera" length category, and as the name implies, they are for formal dress--at least that's what I infer from this:
Add elegance to your formal gown or blouse with an opera-length necklace. At 28 to 34 inches, it draws the eyes down to your chest and away from your neck. This is a good thing if your bosom is more attractive than your chin, but wear a good bra!
Speaking of which, (ahem), if one is well endowed, one should not wear necklaces longer than 22" (I think that's what
it says). If one is full figured, one should wear "chunky" necklaces (and jewellery in general. I've heard this before). If your neck is thick, the folks at
this site recommend staying away from shorter necklaces and opting for graduated, opera or rope (over 45" long) lengths.
This site also raises some interesting issues. If you don't want to draw attention to your bust line, wear necklaces either very short or very long. If you do want to draw attention, then a pendant or a knot right at the breastbone should do it.
Other questions they raise are so basic, I don't know why I've never thought of them before.
1) Do I want the necklace to contrast with my outfit and thus be a focal point or create a serene monochromatic look? Good question.
2) Do I want the focus on the jewellery or the clothes? (Colour is only one way to create contrast. One could contrast textures as well.)
3) Do I want it to sit on my skin or on the shirt? (I don't know. What's the effect of each?)
I decided to go Windows(tm) shopping to find a necklace for this turtleneck:

Using the model in the post below, I decided I wanted the necklace to fall between 24 and 30." This was not an easy length to find!
I like this one: it's pretty. Maybe it's a bit too delicate and fussy for a turtleneck. It is 48" long, so one can wrap it double.

As I looked at Google page after Google page, I realised I have a taste for hand crafted jewellery, as opposed to mass marketed or "rarefied" high priced "gem stone" pieces. I love beads and pendants. I love glass, crystal; sparkly things. And then I saw something from Etsy which led me to this:

It's from
GlowNGlass. It measures 1"x2" on a 24" silver cord.
I think I'm in love. I can certainly make beautiful, one of a kind, artisan crafted jewelery a focal point with casual clothing when it's like this. And it's only $40.00. Thank gawd for Etsy.
ps. I've figured out that all those silver necklaces I own are "snake chains."