In lesson 1 we said a price is just the spot where supply meets demand, and that the only honest way to explain any price move is to ask “what happened to supply, and what happened to demand?” Nice slogan. But to actually use it, you need to see the two things it names — and they’re not points or numbers. They’re curves: whole relationships between price and quantity, drawn on a graph.
This lesson draws those two curves properly and teaches the laws that bend them. Everything later in the course — equilibrium, shortages, taxes, price controls — is just these two lines doing things. Get them right and the rest is almost easy.
Before you read — take a guess
Guess before you read: when the price of a thing goes UP (everything else held fixed), what happens to the quantity people want to buy?
Reading a market graph — the convention
Before we draw anything, let’s agree on the grid we’re drawing on. A market graph has two axes:
- Price runs up the vertical axis (the -axis).
- Quantity — how many units, per week, per month, whatever — runs along the horizontal axis (the -axis).
Think of it like a thermostat with memory. A curve on this graph is a complete answer to one question, asked at every possible price: “At THIS price, what quantity?” Pick a height (a price), slide across to the curve, drop down to the axis, and read off the quantity. One line, infinitely many price-and-quantity pairs.
That “pick a price, read a quantity” move is the whole skill. A curve is not a single dot — it’s the menu of dots.
Here’s the bit that trips up every newcomer and even mildly annoys physicists: price is the thing we control in our heads, yet it sits on the vertical axis. Usually the thing you vary (the “cause”) goes on the horizontal axis. Economists do it backwards. Why? Blame Alfred Marshall, the British economist who popularized these diagrams in 1890 and just… drew them that way. The convention stuck, and a century of textbooks later we’re not changing it. Don’t overthink it — price is up, quantity is across, done.
One graph, two curves, same axes
Both the demand curve and the supply curve live on the exact same axes — price up, quantity across. That’s what lets them cross. The point where they intersect is the famous equilibrium, and it’s the entire subject of lesson 3. For now we just want to draw each line correctly.
When to use it
Reach for a market graph whenever you’re reasoning about a whole market — all the buyers and all the sellers of one good — rather than one person’s single choice. If you only care about “should I buy this one coffee?”, that’s an opportunity-cost decision (lesson 0’s territory). The moment you ask “what’s the price of coffee, and why?”, you’ve zoomed out to the market, and the two-curve graph is the right tool.
The Demand curve & the Law of Demand
Demand is the relationship between the price of a good and the quantity buyers are willing and able to purchase — the whole line, every price-and-quantity pair at once. Drawn on our graph, it slopes downward: low on the left isn’t the point; the point is that as you read from a high price down to a low price, the quantity buyers want grows.
That downward slope has a name: the law of demand — as price falls, quantity demanded rises (and vice versa), holding everything else constant. It is one of the most reliable patterns in all of economics. Three forces drive it, and you’ll recognize the first one from the prerequisite:
- The substitution effect (opportunity cost rising). When coffee gets pricier, coffee’s opportunity cost — what you give up to buy it — goes up relative to tea, energy drinks, or just staying tired. So some buyers substitute away to the cheaper alternative. Higher price → more attractive substitutes → fewer buyers. This is opportunity cost from lesson 0, wearing a market-sized hat.
- The income effect. A higher price means your fixed budget simply buys less. If your weekly $30 coffee budget meets a price hike, you can afford fewer cups even if you’d love to keep buying — you’ve effectively gotten poorer, so you pull back.
- Diminishing marginal value. Your first slice of pizza is worth a lot — you’re hungry. The fifth slice? Meh. Each additional unit is worth less to you than the one before, so you’ll only buy more units if the price drops to match their lower value. Stack everyone’s “fifth slice” together and the whole market only soaks up big quantities at low prices.
Here’s a worked demand schedule — a table that lists the quantity demanded at each price. It’s the same curve, just written as rows instead of a line:
| Price per cup | Quantity demanded (cups/day) |
|---|---|
| $5 | 20 |
| $4 | 40 |
| $3 | 60 |
| $2 | 85 |
| $1 | 120 |
Read it top to bottom and watch the price fall while the quantity climbs — $5 buys only 20 takers, but at $1 you’ve got 120. Plot those five dots and connect them and you’ve drawn a downward-sloping demand curve. That’s the law of demand you can hold in your hand.
Where the curves cross
The two curves, live
Drag demand and supply. The price settles where the two curves meet — push either one and watch the equilibrium move.
Free market: the price clears at $5 and 5 units change hands — the one point where the amount wanted equals the amount supplied.
Pitfall: 'demand' is the whole curve, NOT one number
This is the classic mix-up, and it matters for the rest of the course. Demand = the entire price-quantity relationship — the whole downward-sloping line. Quantity demanded = a single point on that line, the amount at one specific price.
So when a price rises and people buy less, demand did not fall — you just slid to a different point along the same curve. That’s a change in quantity demanded. Demand only “changes” when the whole curve shifts (a new fad, more income, a cheaper substitute) — and that’s a lesson 4 topic. Mix these up and you’ll explain price moves backwards.
Drill the distinction: fill in the two blanks.
Pick the right option for each blank, then check.
The entire downward-sloping line — every price-and-quantity pair at once — is called . The single amount buyers want at one specific price, like 60 cups at $3, is called .
When to use it
Lean on the law of demand whenever you want to predict how buyers react to a price change with nothing else moving — a sale, a price hike, a tax passed to the shelf. It tells you the direction (price up → quantity demanded down) with near-certainty. What it does not tell you is how much — that’s about how steep the curve is (elasticity, much later). Direction first; magnitude later.
The Supply curve & the Law of Supply
Now flip to the other side of the market: the sellers. Supply is the relationship between price and the quantity producers are willing and able to sell — again, the whole line. On the same axes it slopes upward: as you read from a low price up to a high price, the quantity offered for sale grows.
That upward slope is the law of supply — as price rises, quantity supplied rises (and vice versa), holding everything else constant. Why would sellers offer more just because the price is higher? Two reasons, and the second is — surprise — opportunity cost again:
- Higher prices cover higher marginal costs. The cheapest cups to make get made first. Squeezing out extra cups means paying overtime, buying pricier beans, running an older machine — each additional cup costs more to produce than the last (rising marginal cost). Producers only bother making those costly extra units if the price is high enough to cover them. So more units appear only at higher prices.
- Higher prices lure producers away from alternatives (their opportunity cost). Picture a farmer with one field who can plant corn or wheat, not both. If the price of wheat climbs, the opportunity cost of planting corn — the wheat money she’d forgo — rises. So she switches her field to wheat, and the quantity of wheat supplied to the market goes up. Every producer faces some version of this “what else could I be making with these resources?” question, and a higher price tilts the answer toward this good.
Here’s a worked supply schedule for the same coffee market:
| Price per cup | Quantity supplied (cups/day) |
|---|---|
| $1 | 20 |
| $2 | 40 |
| $3 | 60 |
| $4 | 75 |
| $5 | 90 |
Read top to bottom: as the price climbs from $1 to $5, sellers ramp from 20 cups to 90. Plot and connect the dots and you’ve drawn an upward-sloping supply curve — the law of supply, made of rows.
At $3, buyers want 60 cups and sellers offer 60 cups — they match exactly. That price where the two curves agree is the equilibrium, the natural resting point of the market, and it’s the whole story of lesson 3. We’re not solving it yet — just notice the two tables were built to cross. That’s the punchline the rest of the course unpacks.
Same trap, supply side
The “whole curve vs. single point” distinction applies here too. Supply = the entire upward-sloping line. Quantity supplied = the amount at one specific price. A price rise that makes sellers offer more is a change in quantity supplied (a move along the curve) — not an increase in supply. Supply itself shifts only when costs, technology, or the number of sellers change. Same logic as demand, mirror image.
Using the supply schedule above: the price rises from $2 to $4. Which statement is exactly right?
When to use it
Reach for the law of supply to predict how sellers respond to a price change — will a price spike pull more product onto the shelves? (Yes, generally.) It’s the engine behind why shortages tend to fix themselves: a high price doesn’t just ration buyers, it recruits sellers. As with demand, it nails the direction (price up → quantity supplied up); how fast sellers can ramp up is a magnitude question for later.
Why both curves are really opportunity cost
Step back and notice the symmetry. We gave the two laws three-and-two reasons, but underneath, both slopes are the same idea you met in the prerequisite: opportunity cost.
- On the demand side, a higher price raises the opportunity cost of buying — the buyer gives up more of everything else (other goods, future spending) to get this one. So buyers retreat as price rises. The curve slopes down.
- On the supply side, a higher price raises the opportunity cost of not selling — or equivalently, it makes this good worth the resources a producer would otherwise spend elsewhere (the corn-vs-wheat field). So sellers pile in as price rises. The curve slopes up.
Price is the dial that sets opportunity cost for everyone in the market at once. Turn it up and buyers feel a rising cost (they leave) while sellers feel a rising reward (they arrive). The two curves slope in opposite directions because the same rising price is a cost to one side and a lure to the other. That’s not two unrelated rules to memorize — it’s one rule, opportunity cost, viewed from both ends of the transaction.
The one sentence to keep
Demand slopes down and supply slopes up because a higher price makes buying cost more and makes selling pay more — and “cost” here is always opportunity cost, the thing you give up. Two curves, one idea.
Check yourself: the two curves
On a standard market graph, what goes on the vertical (y) axis?
Check your answer to continue.
Where this goes next
You can now draw both curves and say why each one bends the way it does — and you’ve seen, in the two schedules, that they were quietly built to cross at $3 / 60 cups. That crossing point is not an accident; it’s where the whole market wants to be. In lesson 3 — Equilibrium, we put the two curves on the same graph and find that intersection: the one price where the quantity buyers want exactly equals the quantity sellers offer, with no shortage and no glut. Everything you just learned about why each curve slopes the way it does is what makes that meeting point stable. See you where the lines cross.