A must in every store cupboard, and although you can buy loose olives at deli counters (and it's good to buy small amounts to find which varieties you like), always have them tucked away in the larder in jars or tins as well.
That means you can be spontaneous whenever you want to use olives in cooking. I like to have two kinds available: Greek calamata olives, which are quite large, and the tiny purple-brown Provençal ones, which are good for garnishes. Try to buy good-quality olives, and avoid the pitted ones, which are not the best. An olive pitter will make removing the stones fairly easy.
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