*Hypothesis:* compound nouns that are unhyphenated single words can be pluralized by adding an “s” to the noun root *only* when they consist of a noun + preposition.

This is a follow-up to an [earlier question](https://english.stackexchange.com/questions/8450/words-that-are-pluralized-in-the-middle)

Numerous examples are given in the Answers to that Question listing compound nouns that pluralize in the middle by adding an “s” to the *head* or *root* noun, such as **sons-in-law** and **secretaries general**.

However, other than **passersby**, (the word posed in the question and the word that got me thinking, before I found that question), and **standersby** (for which I cannot find an online definition), all of the examples in that Q&A, and those that I can think of or find, are either hyphenated or written as an unhyphenated noun phrase.

Virtually all of these spaced or hyphenated examples are either *noun + adjective* or *noun + prepositional phrase*. The only exception in the examples from the earlier Question is **[spoonsful](http://dictionary.cambridge.org/dictionary/american-english/spoonful)**, but this seems to be a less favored variant of **spoonfuls** (although Microsoft considers *spoonfuls* incorrect and *spoonsful* correct).

There are other single word, unhyphenated compound constructions that serve as nouns which consist of a verb form + preposition which pluralize the usual way, such as

> 
> shutouts
> 
> 
> takeovers
> 
> 
> flybys
> 
> 
> 

Is it possible that the reason for the **passersby** type construction (plural noun root, no space, no hyphen) is the particular combination of noun + preposition? This appears to be a rather rare construction.

I have reviewed word lists in [onelook.com](http://onelook.com/) for any construction with a [word]+s+preposition by searching \*sby and other \*s[preposition] combinations. (I could not search \*s+in because of the volume.)

The only other examples I have found are **strikesthrough** and **sneaksby**. The only reference to **strikesthrough** is a [Wiktionary](http://en.wiktionary.org/wiki/strikesthrough) entry which considers the overall word to be a verb, but several of the listed cites appear to be typos. **Sneaksby** is actually a singular form.

One or two examples are a poor way to make a rule, but could it be that this particular plural form is limited to this particular parts-of-speech structure?