Theoritical aspects

Plurals and Possessives
Most dictionaries will give variant of compound plurals. When you have more than one truck fuddled with sand, do you have several truck fuddled with sand, do you have several truckfuls or trucksful? The dictionary will give you both, with the first spelling usually preferred. (And the same is true of teaspoonfuls, cupfuls, etc.) The dictionary will help you discover that only one spelling is acceptable for some compounds –like passersby.
For hyphenated forms, the pluralizing –s is usually attached to the element that is actually being pluralized: daughters-in-law, half-moons, mayors-elect. The Chicago manual of Style says that “hyphenated and open compound are regularly made plural by the addition of the plural inflection to the element that is subject to the change in number” and gives as examples “father-in-law”, sergeants-in-arms”, “doctors-in-arms”, “doctors of philosophy” and “courts martial”. The NYPL Writer`s Guide puts this way: “the most significant word – generally the noun-takes the plural form. The significant word may be at the beginning, middle, or end of the term”. And then we get examples such as “attorney at law”, “bills of fare”, “chiefs of staff”, “notaries public”, “assistant attorney general”, “higher ups”, “also-rans”, and “go-between”.
As a general rule, then the plural form of an element in a hierarchical term belongs to the base element in the term, regardless of the base element`s placement:

• 
  first sergeant
  
• 
  sergeants major
  
• 
  sergeant first class
  
• 
  colonel generals(Russian)
  
• 
  lieutenant generals
  
• 
  lieutenant colonels
  
• 
  apprentice Journeyman and master mechanics
  
• 
  deputy librarians
  
• 
  deputy assistant secretaries of state
  

The possessive of a hyphenated compound is created by attaching an apostrophe –s to the end of the compound itself: my daughter-in-law`s car, a friend of mine`s car. To create the possessive of pluralized and compounded forms, a writer is wise to avoid the apostrophe –s form and use an “of phrase (the “post genitive”) instead: the meeting of the daughter-in-law, the schedule of half-moons. Otherwise, the possessive form becomes downright weird: the daughter-in-law`s meeting, friends of mine`s cars.
One of the most difficult decisions to make about possessives and plurals of compound words occurs when you can`t decide whether the first noun in compound structure is acting as a noun that ought to be showing possession or as what is called an attribute noun, essentially an adjective. In other words, do we write that: I am going to writer`s conference or to a writers` conference? The Chicago Style Manual suggests that if singular nouns can act as attributive nouns – city government, tax relief – then plural nouns should be able to act as attributive nouns: consumer group, teachers union. This principle is not universally endorsed, however, and writers must remember to be consistent within a document.
The section doesn`t speak to the matter of compounded nouns as “Professor Villa`s and Professor Darling`s classes have been filled.”
Compound nouns can be further subdivided into four groups according to semantic criteria. Consider first the examples beehive, armchair, redskin, highbrow, maidservant. These can all be divided into a second element, which is the grammatical head (this is the element marked for number, and also, in languages which have grammatical gender, the element which determines the gender of the compound) and a first element which id the modifying element (which is not marked for number and doesn`t determine gender). In the first two examples, the compound is a hyponym of the grammatical head: a beehive is a kind of hive; an armchair is a kind of chair.
This type of compound is termed an endocentric compound. In the second two examples, the compound is not a hyponym of the grammatical head: a redskin is not a type of skin, nor is a highbrow a type of brow.
This type of compound, termed an exocentric compound (or sometimes a bahuvrihi compound, using the Sanskrit terminology), is a hyponym of some unexpected semantic head (‘person ‘ in both the examples given here). Since the semantic head is unexpressed in such compounds, the compound is frequently seen as metaphorical or synecdoche.
Thirdly, maidservant is a hyponym of both maid and servant: a maidservant is a type of maid and also a type of servant.
This type of compound is called an appositional compound.
The final division of compound nouns is exemplified by Alsace-Loraine and Rank-Hives. Here is not always clear which element is the grammatical head and the compound is not a hyponym of either element, but the elements name separate entities which combine to form the entity denoted by the compound. This type of compound is normally given the Sanskrit name of dvandva, although the English term copulative is also used to describe them.
These semantic divisions interact with syntactic divisions according to the form class of whole compound (compound noun, compound adjective, etc.) and the form classes of the individual elements in the compounds, so that redskin, as well as being an exocentric compound, is a compound noun made up of an adjective and a noun, and armchair is an endocentric compound noun made up of two nouns.