Quotation marks, also called 'inverted commas', are of two types: single and double. British practice is normally to enclose quoted matter between single quotation marks, and to use double quotation marks for a quotation within a quotation:
The oceans absorb the sun's heat, transferring it to the atmosphere and distributing it around the world via the ever-moving ocean currents. This drives global weather patterns and acts as a heater in the winter and an air conditioner in the summer.
1 Density currents are of considerable practical importance. 2  Turbidity currents have been investigated in the laboratory and have been observed directly in lakes and ocean basins. 3  Ocean waters with the greatest density have their origins at high latitudes and in marginal seas.