Electronic invoicing allows businesses to exchange financial information in structured formats that their systems can automatically process. But if every organisation uses different accounting software, how does an invoice travel reliably from one system to another? The answer lies in the networks and platforms connecting businesses, and the different models used to manage those exchanges.
What are corners in e-invoicing?
One of the challenges with electronic invoicing is that businesses rarely operate on identical financial systems. A manufacturer might use an enterprise resource planning (ERP) solution, while its suppliers use a mixture of accounting platforms, specialist software and other ERP systems.
For those organisations to exchange structured invoices, there needs to be a reliable method of transferring information between their respective systems. This is where the concept of e-invoicing corners becomes important.
In simple terms, a corner represents a participant or intermediary involved in exchanging an electronic invoice. Depending on the model, those participants might include the supplier, buyer, service providers and tax authorities.
Three-corner, four-corner and five-corner models describe different ways of arranging those relationships. The terminology can vary between countries and technical frameworks, but understanding the underlying principles makes the different approaches much easier to follow.
The three-corner model: A centralised exchange
The three-corner model describes an arrangement in which a central intermediary sits between the supplier and buyer.
Instead of communicating directly, the supplier submits its electronic invoice to the central platform, which processes the information and delivers or makes it available to the recipient.
The exchange can therefore be understood as moving from the supplier to a central platform and then to the buyer.
Where that central platform is operated by a government authority, it can perform an additional regulatory function, checking invoices against specific requirements before allowing them to proceed.
Italy provides a useful example of this centralised approach. Since 2019, most domestic business-to-business invoices in Italy have been required to pass through the country’s Sistema di Interscambio (SDI).
When an invoice is submitted, SDI performs prescribed checks before routing it to the recipient. This gives the Italian tax authority visibility over the information being exchanged and allows certain errors to be identified before an invoice reaches its destination.
The advantage of a centralised model is that businesses have a clearly defined route for exchanging invoices. However, they must also ensure that their systems can communicate with the platform and meet its technical and regulatory requirements.
It is worth noting that the term three-corner model is not used identically everywhere. Some technical descriptions apply it to arrangements involving a single shared service provider rather than a government-operated exchange. What matters in this context is the centralised nature of the intermediary.
The four-corner model: How Peppol works
The four-corner model uses a different approach. Instead of relying on one central platform to exchange every invoice, businesses connect through a network of interoperable service providers.
This is the principle behind Peppol, an international framework that allows organisations to exchange structured electronic business documents using common standards.
To understand how it works, consider a supplier sending an invoice to a customer. The supplier creates the invoice in its accounting or ERP system, but rather than sending it directly to the customer, the information is passed to its chosen service provider, known as an access point.
The supplier’s access point handles the transmission of the invoice across the network and delivers it to the buyer’s access point. The buyer’s access point then passes the information to the receiving organisation, where it can be processed by its financial system.
These four participants form the four corners: the supplier, the supplier’s access point, the buyer’s access point and the buyer. What makes the model particularly useful is that the two businesses do not need to use the same software or service provider.
As long as both organisations are appropriately connected to the network and support the relevant document requirements, they can exchange electronic invoices through their respective access points.
Why is Peppol different from traditional EDI?
Electronic data interchange (EDI) has enabled businesses to exchange structured information for decades, so the concept of transferring machine-readable invoices is not entirely new.
However, traditional EDI implementations can involve individual technical arrangements between trading partners, particularly when organisations use different formats, systems or communication methods.
For a business working with hundreds of suppliers, establishing and maintaining these connections can become complex. Peppol addresses this challenge through a shared framework for identifying participants, exchanging supported documents and connecting service providers.
Rather than developing a separate integration for every customer or supplier, a business can use its chosen Peppol access point to communicate with other compatible participants.
This does not mean every trading relationship becomes completely effortless. Businesses still need accurate partner information, suitable document formats and systems capable of processing the information they receive.
However, the common network removes much of the complexity associated with establishing individual connections. For organisations trading with multiple businesses or operating internationally, that interoperability can provide considerable advantages.
The five-corner model: Bringing tax authorities into the process
The five-corner model extends the four-corner arrangement by introducing an additional participant: the tax authority.
The supplier and buyer continue to exchange invoices through their respective service providers, but relevant invoice or transaction information is also communicated to the government. This additional connection is particularly important in countries introducing digital tax reporting requirements.
Instead of waiting for periodic VAT returns, tax authorities can receive information much closer to the time a transaction takes place, helping them monitor compliance and identify discrepancies.
France provides a useful example of an e-invoicing framework that incorporates this principle. Under the French model, businesses exchange qualifying electronic invoices through government-approved platforms. These platforms are also responsible for transmitting prescribed invoice, transaction and payment information to the tax administration. In other words, the systems responsible for exchanging invoices between businesses also support the government’s reporting requirements.
An important distinction is that the fifth corner does not necessarily mean the government must approve every invoice before the buyer receives it. In some systems, the tax authority receives the required information alongside the exchange. In others, invoices must undergo a prescribed validation or clearance process before they satisfy the relevant legal requirements.
The model therefore describes the additional involvement of the tax authority, rather than establishing a universal requirement for government approval.
Which e-invoicing model is better?
There is no single model that is inherently better for every country or business. Each reflects different priorities around interoperability, regulation and control.
Centralised exchanges can give governments a direct role in validating and distributing invoices, while interconnected networks such as Peppol place greater emphasis on enabling businesses to communicate through their chosen service providers.
Five-corner arrangements seek to combine this decentralised exchange with additional tax reporting. For businesses, the practical question is often less about choosing a preferred model and more about understanding which arrangements they need to support.
An organisation operating across several countries may encounter centralised platforms in one market, Peppol-based exchanges in another and additional reporting requirements elsewhere.
Its financial systems must therefore be capable of accommodating the requirements of the jurisdictions in which it operates.
Understanding the corner models is an important first step because it explains how invoices move between organisations. But the network through which an invoice travels is only one part of the picture.
The next question is why governments are introducing different requirements, and how much involvement they expect to have in the invoicing process.
That brings us to the distinction between post-audit, real-time reporting and clearance, and the growing number of mandatory e-invoicing frameworks being introduced around the world.
