Not all plurals are plural

In our Hebrew Reading Group today we came across what appeared to be a confusing grammatical situation.  In proverbs 1:9 the text is:
כִּי לִוְיַת חֵן הֵם לְרֹאשֶׁךָ וַעֲנָקִים לְגַרְגְּרֹתֶיךָ׃
   The confusing part is that the last word לְגַרְגְּרֹתֶיךָ if taken by it's form means "on your necks".  This doesn't agree with all the previous verses and the first half of verse 9 where the addressee is spoken of in the singular.  I looked this up in Gesenius' Hebrew Grammar and in "An Introduction To Biblical Hebrew Syntax" by Waltke and O'Connor.  It turns out that neck (or outside of the neck as this Hebrew word means) is a plural of extension.  Gesenius calls it "local extension" and Waltke calls it just "extension".  It is found in section 124 of Gesenius' grammar. Here is what he writes:

The plural is by no means used in Hebrew solely to express a number of individuals or separate objects, but may also denote them collectively. This use of the plural expresses either (a) a combination of various external constituent parts (plurals of local extension), or (b) a more or less intensive focusing of the characteristics inherent in the idea of the stem (abstract plurals, usually rendered in English by forms in -hood, -ness, -ship). A variety of the plurals described under (b), in which the secondary idea of intensity or of an internal multiplication of the idea of the stem may be clearly seen, is (c) the pluralis excellentiae or pluralis maiestatis.

 (a): Plurals of local extension denote localities in general, but especially level surfaces (the surface-plural), since in them the idea of a whole composed of innumerable separate parts or points is most evident.

Some of the examples he gives are the Hebrew words for sea, face, back, neck (a different word than the one in Prov 1), place around the head, place around the feet, place on the other side of a river, bed... etc..  We spoke about the Hebrew word for blood (because it also occurs here and is always plural) and it appears that blood is also a plural of local extension.

     Here is what Waltke and O'Conner have in their book- "Plurals of Extension indicate that the referent of the noun is inherently large or complex;the plural quality is the result not of countable multiplicity  but of a multiplicity that is nonetheless perceived as real."  

     They go on to give some examples of how we use this in English.  Literal words like...  waters, guts...  metaphorical words like brains, wits, looks...   and abstract words thanks, amends, auspices"  It is interesting to me how we use these words in English without thinking about the plural forms because we are native speakers of the language...  we certainly are not thrown for a loop like we were when we encountered them in our Hebrew reading.