 For most singular nouns the ending "'s" is added; e.g., "the cat's whiskers".When the noun is a normal plural, with an added "s", no extra "s" is added in the possessive; so "the neighbours' garden" (there is more than one neighbor owning the garden) is standard rather than "the neighbours's garden".Compound nouns have their singular possessives formed with an apostrophe and an added "s", in accordance with the rules given above