Possessives and Attributives

Q. I’m on a team editing kids’ textbooks. One book includes a poster showing shapes (circle, square, triangle). Should this be referred to as a shapes poster? Is it an example of the genitive case 4 (at CMOS 5.20), requiring an apostrophe: shapes’ poster? If not, is it a temporary compound noun? Could it be written either way, based on personal preference? Does genitive case 7 help at all? A poster of shapes = shapes’ poster.

A. No apostrophe is needed because shapes is an attributive noun. “A shapes poster” is grammatically akin to “a commodities trader” and “a weapons dump” in containing a noun (singular or plural) that is used attributively as an adjective (shapes, commodities, weapons). You can read about attributive nouns at CMOS 5.24, “Nouns as Adjectives.”