Movement along the demand curve, or a shift of it?

Students mix these up in definitions, in diagrams and in the words they use. One question sorts it out.

Did the good’s own price change?

• Yes → a movement along the same curve. The board’s words are change in quantity demanded: expansion when price falls, contraction when it rises.
• No, something else changed → the whole curve shifts. This is a change in demand: an increase (rightward) or a decrease (leftward).

What shifts the curve

• Income — for a normal good, higher income shifts demand right; for an inferior good, left.
• Price of a related good — a dearer substitute shifts demand right; a dearer complement shifts it left.
• Tastes, preferences and expectations.

In the diagram

Label both axes (price; quantity demanded), name each curve (D₁, D₂), mark the old and new points, and draw the arrow the right way. A shift drawn as a movement along one curve loses the diagram marks even when the explanation is right.

The same logic, with the same words, applies to supply.