Contents
Economics & Markets
Section 1
The Core Idea
Two products are complements when consuming more of one increases demand for the other. They are substitutes when consuming more of one decreases demand for the other. The distinction sounds academic. It determines the structure of entire industries. Gillette sells razors at cost and makes money on blades. Printers are sold cheap; ink cartridges are expensive. The razor-blade model works because razors and blades are complements — buying one creates demand for the other. The same logic explains why Apple subsidises third-party app development and why Amazon built Prime: each complements the core product and increases its value.
Substitutes operate in reverse. When Uber entered a city, taxi demand fell. When Netflix grew, cable subscriptions collapsed. The relationship is zero-sum: more of one means less of the other. Strategists who mistake complements for substitutes — or vice versa — make catastrophic errors. Microsoft treated Netscape as a substitute in the 1990s browser wars and nearly lost the platform. Google treats Android as a complement to search: the more Android devices, the more search queries, the more ad revenue. The same product can be a complement in one relationship and a substitute in another. The iPhone is a complement to the App Store and a substitute for the digital camera.
The cross-price elasticity of demand formalises the relationship. If the price of good A rises and demand for good B increases, A and B are substitutes — consumers switch from A to B. If the price of good A rises and demand for good B falls, they are complements — consumers buy less of the bundle. This single metric predicts merger outcomes, antitrust rulings, and competitive dynamics. Regulators block mergers between close substitutes. They scrutinise vertical integration between complements. Founders who map their product's complement and substitute landscape before competitors do gain a structural advantage.
Section 2
How to See It
The pattern appears wherever products are consumed together or in place of each other. The diagnostic: does more of X make people want more of Y, or less?
Technology
You're seeing Complements & Substitutes when a cloud provider offers free credits to startups — AWS, Google Cloud, and Azure all subsidise early-stage companies. The cloud is a complement to the startup's product: more startups building more applications means more compute demand. The provider that captures the complement relationship early locks in the customer before they scale. The same logic explains why Microsoft gives away developer tools: Visual Studio Code and GitHub are complements to Azure. More developers using Microsoft's free tools means more workloads on Microsoft's paid infrastructure.
Retail
You're seeing Complements & Substitutes when a grocery store places milk at the back of the store. Milk is a complement to nearly everything else in the cart — bread, cereal, cookies. Forcing customers to walk past every aisle to reach it increases the probability they add complementary items. The store is not optimising for milk sales. It is optimising for the basket. The same logic explains loss leaders: Costco sells rotisserie chickens at a loss because chicken is a complement to the rest of the shopping trip. The chicken brings you in. The margin is in everything else.
Media
You're seeing Complements & Substitutes when Spotify invests in podcasts. Music and podcasts are substitutes for listening time — both compete for the same ear. But podcasts are complements to the Spotify subscription: more podcast content increases the value of the monthly fee, reducing churn. The strategic move is to own both sides of the substitute relationship (competing for attention) while making them complements to the business model (both increase subscription value). Netflix does the same with games: games substitute for viewing time but complement the subscription by increasing perceived value.
Investing
You're seeing Complements & Substitutes when a company acquires a supplier or distributor. Vertical integration between complements can capture value that was previously split across the chain. Disney's acquisition of Fox added content (complement to distribution) and removed a competitor for licensing rights. The risk: regulators may treat the integrated firm as having excessive market power over complements. The opportunity: controlling complements lets you price the bundle to maximise total surplus rather than optimising each piece separately.
Section 3
How to Use It
Map your product's complement and substitute landscape before making pricing, partnership, or acquisition decisions. The same move can be brilliant or disastrous depending on which relationship you're optimising.
Decision filter
"Before any strategic move, ask: is this product a complement (more of it increases demand for mine) or a substitute (more of it decreases demand for mine)? Complements should be subsidised, bundled, or acquired. Substitutes should be differentiated, blocked, or acquired to neutralise."
As a founder
Identify your complements and subsidise them. If your product is a platform, the applications built on it are complements — make it easy and cheap for developers to build. If your product requires adoption of a new behaviour, the tools that enable that behaviour are complements — partner with them or build them. The mistake: treating a complement as a revenue source and taxing it. That turns a complement into a substitute — developers will build for the platform that doesn't tax them. The win: make your complements so valuable that substitutes become irrelevant. Nobody substitutes away from the iPhone because the App Store's complement ecosystem is too valuable to leave.
As an investor
Evaluate moats through the complement-substitute lens. A company with defensible complements — proprietary content, exclusive distribution, integrated tools — has a structural advantage. A company whose product is easily substituted — commodity offering, no complement lock-in — will face margin compression. The best investments often sit at the intersection: a product that is a complement to a growing ecosystem (riding the tide) while having few close substitutes (defensible position). Tesla's Supercharger network is a complement to Tesla ownership; it also reduces the substitute threat from other EVs by making Tesla ownership more valuable.
As a decision-maker
When evaluating partnerships or acquisitions, classify the relationship. Acquiring a complement integrates the value chain and can justify premium pricing for the bundle. Acquiring a substitute eliminates a competitor but may attract antitrust scrutiny. The regulatory test often hinges on whether the merged entity would have power over complements (potentially abusive) or over substitutes (reduced competition). Frame your strategy in the language regulators use — it will clarify your own thinking.
Common misapplication: Assuming the relationship is static. Complements can become substitutes when technology or behaviour shifts. The newspaper and the radio were complements in the morning routine — read the paper, listen to the news. The smartphone made them substitutes — both compete for the same attention. The founder who treats today's complement as permanent will be surprised when it flips.
Section 4
The Mechanism
Section 5
Founders & Leaders in Action
Jeff BezosFounder & CEO, Amazon, 1994–2021
Bezos understood that Prime was not a standalone product — it was a complement to everything Amazon sold. Free two-day shipping made every purchase on Amazon more valuable. The more you bought, the more valuable Prime became. The more valuable Prime became, the more you bought. Bezos reportedly told his team that the goal was to make Prime so valuable that "it would be irresponsible not to be a member." He subsidised the complement aggressively: Prime Video, Prime Music, Prime Reading, and prescription discounts were all added at a loss to increase the value of the membership. The strategy was to own the complement relationship so completely that substitutes — other retailers, other streaming services — became irrelevant. By 2024, Prime had over 200 million members, and the complement logic had created a moat that no competitor could replicate.
Steve JobsCo-founder & CEO, Apple, 1997–2011
Jobs treated the App Store as a complement to the iPhone, not a revenue centre. Apple took 30% of app sales, but the primary goal was to make the iPhone indispensable by ensuring it had the best complement ecosystem. When developers complained about the cut, Jobs held the line — but he also invested heavily in developer tools, documentation, and distribution. The insight: the iPhone's value was the sum of the device plus every app on it. Taxing complements too heavily would shrink the ecosystem; subsidising them would grow it. By 2011, the App Store had over 500,000 apps. The complement strategy had made the iPhone the most valuable product in consumer technology — not because of the hardware alone, but because of the ecosystem of complements that made switching to a substitute (Android) a downgrade for many users.
Section 6
Visual Explanation
Complements & Substitutes — Demand for B moves in opposite directions depending on whether A and B are consumed together or in place of each other.
Section 7
Connected Models
Reinforces
Network Effects
Network effects amplify complement logic. When more users make a product more valuable, each user is a complement to every other user. The platform that attracts the most complements (developers, content, users) wins. Complements and network effects compound: more complements increase value, which attracts more users, which attracts more complements.
Reinforces
Bundling and Unbundling
Bundling works when the bundled items are complements — consuming one increases the value of the others. Unbundling attacks bundles by offering a substitute for the most valuable component. The bundler's defence is to make the components such strong complements that the unbundled alternative feels incomplete.
Tension
Creative Destruction
Creative destruction often arrives as a substitute — a new technology that replaces the old. The incumbent's complement strategy (building an ecosystem) can slow substitution, but cannot stop it if the substitute offers a fundamentally better value proposition. The tension: complements extend the life of a product; substitutes shorten it.
Tension
Platform Business Model
Platforms depend on complementors — third parties who add value. But complementors can become substitutes if they capture too much value or if they build their own platform. The platform must balance making complements valuable enough to attract them while retaining enough value to prevent them from substituting the platform entirely.
Leads-to
Switching Costs
Complements create switching costs. The more complements you use (apps, content, integrations), the higher the cost of switching to a substitute. A product with strong complements is harder to substitute away from — even if a competitor offers a better core product.
Leads-to
Price Elasticity
Cross-price elasticity is the formal measure of complement and substitute relationships. Understanding how demand for your product responds to the price of related products — and vice versa — enables precise pricing and partnership strategy. Elasticity reveals which products are true complements (negative cross-elasticity) and which are true substitutes (positive cross-elasticity).
Section 8
One Key Quote
— Michael Porter, Competitive Strategy (1980)"The intensity of competition in an industry depends on five forces... Substitutes limit the potential returns of an industry by placing a ceiling on the prices firms can charge."
Section 9
Analyst's Take
Faster Than Normal — Editorial View
The complement-substitute framework is underused in startup strategy. Most founders think in terms of competitors (substitutes) but underinvest in identifying and cultivating complements. The best moats are built by owning the complement relationship — making your product so valuable when combined with others that substitutes become irrelevant. Amazon's moat is not retail; it's the complement ecosystem of Prime, logistics, and third-party sellers that makes leaving irrational.
The trap: treating complements as revenue sources. When you tax a complement — high platform fees, expensive add-ons, restrictive terms — you incentivise substitutes. Developers will build for the platform that doesn't tax them. The platform that extracts the most from complementors in the short term often loses them in the long term. The winning move is to subsidise complements until the ecosystem is so valuable that substitutes cannot compete.
One more pattern: the complement-substitute flip. Technology and behaviour shifts can turn complements into substitutes. The smartphone made the camera and the GPS complements to the phone (bundled) and substitutes for standalone devices (replaced). The founder who anticipates the flip — and positions their product on the right side of it — captures disproportionate value.
Section 10
Test Yourself
Is this a complement or substitute relationship?
Scenario 1
A SaaS company that sells project management software is considering acquiring a time-tracking tool. The time-tracking tool would be integrated into the project management workflow, with data flowing between both.
Scenario 2
A streaming service is deciding whether to add a gaming feature. Gaming would compete for the same leisure time as video content. Subscribers would have to choose between watching a show or playing a game.
Scenario 3
A cloud provider offers free tier services to attract developers. The free tier includes compute, storage, and database — enough to build and run a small application. The provider makes money when developers scale up and pay for higher tiers.
Section 11
Top Resources
01
Book
Porter's five forces framework formalised substitutes as a competitive pressure. His analysis of how substitute products limit industry profitability provides the strategic foundation for understanding when to differentiate, when to acquire, and when to build complement ecosystems to reduce substitute threat.
02
Book
Shapiro and Varian apply complement-substitute logic to information goods. Their analysis of how to price complements (often below cost) and how to create lock-in through complement ecosystems is essential for technology strategy.
03
Book
The definitive treatment of how platforms use complementors to create value. The book explains why platforms subsidise one side to attract the other, and how complement relationships drive network effects and winner-take-all dynamics.
04
Book
Standard microeconomics textbooks cover cross-price elasticity of demand — the formal measure of complement and substitute relationships. Understanding the math clarifies when products are true complements (negative cross-elasticity) versus substitutes (positive cross-elasticity).
05
Book
Tirole's treatment of vertical integration, bundling, and complement relationships provides the theoretical foundation for antitrust analysis and strategic pricing. Essential for understanding when regulators will block mergers between complements or substitutes.
Why this matters next
mental modelsNetwork Effects
Complements & Substitutes applied the Network Effects mental model
mental modelsPerceived Value
Complements & Substitutes applied the Perceived Value mental model
mental modelsMarket Power
Complements & Substitutes applied the Market Power mental model
mental modelsScale
Complements & Substitutes applied the Scale mental model
mental modelsChurn
Complements & Substitutes applied the Churn mental model
mental modelsCost
Complements & Substitutes applied the Cost mental model
Frequently asked questions
What is Complements & Substitutes?
Complements & Substitutes is a mental model used for better thinking and decision-making.
How do you apply Complements & Substitutes?
To apply Complements & Substitutes, identify situations where this framework is relevant, then use it as a lens to evaluate your options and decisions. The model is most useful when combined with other complementary mental models.
What category does Complements & Substitutes fall under?
Complements & Substitutes falls under the Economics & Markets category of mental models. Other models in this category can be found on the Economics & Markets hub page.
Why is Complements & Substitutes important?
Complements & Substitutes is important because it provides a structured way to think about problems that would otherwise be approached with intuition alone. Understanding this model helps you avoid common reasoning errors and make better decisions.
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