A question about the conjunction and

[GKanka]

Hi,
One grammar guide teaches that if two modifiers of similar kind refers to the same noun (thing or person) only the first is preceded by an article, while the noun is in the singular (The black and white dress she had on was very becoming); but if they refer to different things the noun is in the plural, with an article preceding each modifier (The black and the white dresses were very becoming). This, as I have understood it, means that, for example, the phrase a/the political, economic, and social sphere implies that the sphere is at once economic, political, and social. But how should I understand (if the above rule really governs the structure) an example where the noun is in the plural but only the first modifier is preceded by an article as it is in a sentence you can read in the CollinsCobuild dictionary--We are doing this work in the context of reforms in the economic, social and cultural spheres. The use of the plural noun means that the three spheres are considered different things by the writer, and thus, the the would have to stand before each adjective like here-- the economic, the social, and the cultural spheres. Does this mean that the rule is wrong or incomplete, or I have misunderstood something?

[5jj]

One grammar guide teaches that if two modifiers of a similar kind refers to the same noun (thing or person) only the first is preceded by an article, while the noun is in the singular (The black and white dress she had on was very becoming); but if they refer to different things the noun is in the plural, with an article preceding each modifier (The black and the white dresses were very becoming).
This is only to avoid ambiguity.Without the second determiner, both dresses could have been black-and-white. Where there is no possibility of ambiguity, a second determiner is not necessary - The French and Russian Ambassadors were at the reception.

-This, as I have understood it, means that, for example, the phrase a/the political, economic, and social sphere implies that the sphere is at once economic, political, and social. But how should I understand (if the above rule really governs the structure) an example where the noun is in the plural but only the first modifier is preceded by an article as it is in a sentence you can read in the CollinsCobuild dictionary--We are doing this work in the context of reforms in the economic, social and cultural spheres.
Three different spheres are clearly meant, and so the second and third determiners can be omitted.

Does this mean that the grammar guide lies and the rule is wrong or incomplete, or I have misunderstood something?

The grammar guide has given an over-simplified version of what happens.