Economics students can use this useful Diver & Surfboard mnemonic to remember that:
The Demand curve (Diver) slopes downward - because divers dive down
The Supply curve '(Surfboard)' slopes upward - because 'Surf's Up'
Price is on the vertical axis - because prices tend to rise (inflation)
Quantity is on the horizontal axis - because quantities are heavy and fall down (thanks gravity)
Finally, P* refers to the Market Equilibrium Price, and Q* refers to the Market Equilibrium Quantity which co-occur where the Demand and Supply curves intersect.
Note additionally that the Demand curve is depicted in blue because demand comprises the preferences of consumers - which are ultimately subjective and hence seem to 'come out of the blue'. Whereas the Supply curve is depicted in red because in the long-run the Supply curve is equivalent to industry-wide production costs, and accounting items that detract from profits (such as production costs) are customarily colored in red. Naturally the intersection of the blue Demand curve and red Supply curve gives rise to a purple equilibrium price (P*) and equilibrium quantity (Q*), just like when mixing these colors in real life.
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