You might buy a printer because it’s reasonably priced.

You find a good deal, compare models, pay, and take it home.

But later, when the ink cartridge runs out, you realize that buying the printer wasn’t entirely a decision made in isolation from something else.

Ink.

The printer alone can’t do what you bought it for. And ink on its own isn’t much use either.

This is where a simple economic idea comes in—one that is all around us more often than we might think: some goods aren’t consumed on their own; we need them alongside other goods to get the benefit we want.

These are complementary goods.

Sometimes, the price of the product alone isn’t what matters

When you’re thinking about buying a printer, it’s only natural to look at its price.

But if you knew that ink was very expensive, you might reconsider.

You might choose another printer.

Or put off buying one.

Or decide that using a print shop would cost you less.

The printer’s price hasn’t necessarily changed.

But the cost of something you need to use with it has.

This is precisely where the distinction between price and demand begins—a distinction explained by Dr. @Faiq Al-Akayleh as part of the determinants of demand in the Managerial Economics course (ECO 506) at Al Yamamah University.

The course includes the prices of other goods among the factors that can change demand, and distinguishes in this context between substitute goods and complementary goods.

What is a complementary good?

The definition is simple:

A complementary good is a good that is used with another good to achieve a particular purpose or benefit.

That is why the relationship between the price of a complementary good and demand for the other good is inverse. If the price of a complementary good rises, demand for the other good falls; if its price falls, demand for the other good rises, all else being equal.

Let’s go back to the printer.

If the cost of ink rises sharply, the printer may become less attractive to some buyers.

Not because the printer itself has become more expensive.

But because using it has become more costly.

This is an important economic point: consumers don’t always buy a product independently of the other products they need to use with it.

Why can demand fall even when the price stays the same?

Suppose a particular printer costs 500 riyals.

But the price of the ink it needs goes up.

The consumer might say:

"The printer is reasonably priced, but it’s no longer practical for me to use."

Here, demand for the printer can fall even though the printer’s own price hasn’t changed.

This differs from the case where we discussed the effect of a rise in the price of the good itself.

In that case, a price change causes movement along the demand curve.

Here, however, a change in the price of a complementary good causes the demand curve for the other good to shift. The course material explains that a rise in the price of a complementary good shifts the demand curve backward, while a fall in its price shifts it forward.

This distinction is essential to understanding economic figures.

Consumers don’t buy the product—they buy the benefit it provides

This might sound philosophical, but it’s highly practical.

You don’t buy a printer because you want a piece of plastic sitting on your desk.

You buy it because you want to print documents.

That’s why anything that significantly affects your ability to use the printer can also affect how attractive the printer itself is.

The same applies to many products.

A phone and a charger.

A car and fuel.

Certain devices and the accessories needed to operate them.

In every case, consumers don’t look only at the primary product; they also consider the cost and benefit of using the products together.

This is where competition gets more complicated

Companies don’t compete only on the price of the product they sell.

Sometimes, a product that seems cheaper at first ends up costing more over time because of the complementary goods it requires.

That’s why, when consumers compare two products, the question may not be:

Which one is cheaper?

It may be:

Which one will cost me less when I actually use it?

That’s a big difference.

The price on the product label is the start of the calculation, not always the end.

What happens if ink gets cheaper?

Let’s reverse the example.

If the price of ink falls, the printer may become more attractive to consumers.

The printer itself hasn’t become cheaper.

But the cost of using it has gone down.

That’s why demand for it may rise.

This is the relationship explained directly in the course material: a fall in the price of a complementary good increases demand for the other good, while a rise in its price reduces demand for it.

This shows how two markets can affect each other even when the goods aren’t competing.

It’s because the goods work together.

The difference between a substitute and a complement

This highlights the importance of distinguishing between two concepts that may seem similar to readers unfamiliar with the subject.

If one product can replace another, they are substitutes.

But if using one depends on the other, or it becomes more useful when paired with it, they are complements.

In the case of substitutes, a rise in the price of one product may push consumers toward the other.

With complements, a rise in the price of one may also pull demand for the other down. This is the fundamental difference presented in the course material between the positive relationship of substitute goods and the negative relationship of complementary goods.

Put simply:

A substitute competes with you.

A complement helps you use the other product.

Why does this matter to businesses?

Because a company selling one product may be affected by the price of another product it doesn’t even make.

If the cost of a complementary good rises, demand for the company’s product may fall.

If the complementary good becomes cheaper, the opposite may happen.

This means that keeping an eye on the market involves more than monitoring the price of the product itself.

The course material also expresses this relationship through the demand function: if the coefficient for the price of another product is positive, this indicates a substitute relationship, while a negative relationship indicates that the products are complements.

For consumers, the question isn’t always, "How much does the product cost?" The more important question may be, "How much will it cost me to use it?"

When the bill isn’t for just one product

In our daily lives, we rarely think about these relationships the way an economist does.

We buy the printer, then the ink.

We buy the car, then what we need to run and use it.

And we treat each purchase as if it were a separate decision.

But the market may see things differently.

The price of one product can change demand for another, even if the second product’s price hasn’t changed.

That’s the idea behind complementary goods, as explained by Dr. Faiq Al-Akayleh in his discussion of the determinants of demand: when the cost of a complementary good rises, the other product may become less attractive and demand for it may fall; when the cost of the complement falls, the product may become more attractive and demand for it may rise.

So the next time you see a product at an appealing price, it may be worth asking one more question before you buy:

How much will the things I need to use with it cost me?

Sometimes, the price printed on the product doesn’t tell the whole story.

Disclaimer: This material was prepared under the supervision of an editor at “Yamamah Insights” with the assistance of artificial intelligence tools for financial education purposes. It is not a recommendation to buy, sell, or hold any security. The views expressed are those of the authors and do not represent the platform’s views.