The apostrophe is used to show possessionSee the glossary definition (The greengrocer's shop) and contractionSee the glossary definition (George, don't do that!).

1. Possession

   Possession here means the use of the genitiveSee the glossary definition case, which signifies belonging. They are various types but the most important are specifiers (pointing to a particular reference) and classifiers (placing in a particular class or group).

   Where the noun is singular, possession is marked by an apostrophe + s. (her daughter's birthday). Where the noun is plural, possession is marked simply by an apostrophe. (her daughters' birthdays – more than one daughter). Where the noun has an irregular plural, possession is marked by an apostrophe + s (her children's birthdays).

   • For example, Einstein’sEinstein’s theory of relativity (theory belonging to Einstein) theory of general relativity got a boost over Newton’sNewton’s theory (also ellipsis here referring back to Einstein’s theory) because it predicted the precise migration of Mercury’sthe closest point of Mercury to the sun closest point to the Sun year after year.
   • Classic examples include the dazzling blue flash of a Morpho butterfly’s wingthe wing of a Morpho butterfly (one butterfly - so apostrophe + s) and the golden mirror-like reflection from a Chrysina beetle, both produced by microscopic layers in the insects’ tissuesthe tissues of insects (plural insects - so s + apostrophe).
   • But at high magnification, the scales’ coloursthe colours of the scales (plural scales - so s + apostrophe) are astonishing: vivid greens, blues and hints of yellow.
    
   

2. Contraction

   • There are also galaxies that don’tcontraction of 'do not' have any particular shape at all.
   • At least, that’scontraction of 'that is' what received wisdom tells us.
   • But exactly why insects are so colourful isn’tcontraction of 'is not' always clear.
    
   

3. It's and Its

   Don't confuse it's and its; they are completely different.

   • It’scontraction of 'It is' typical – you’re waiting at a bus stop for ages, then three buses come along at once.
   • With bread, cheese and tomato sauce as its this is a possessive determiner; there is no contraction so it doesn't need an apostrophe base, pizza might seem like a simple food.
    
   

These examples were sourced from articles in The Conversation: Astronomy and Astrology; Insect colours; Galaxies; Buses Come in Threes; Pizza; Are firstborns really natural leaders?

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