Tangle

The Tangle is the ledger design created for IOTA (IOTA), in which transactions are recorded in a Directed Acyclic Graph (DAG) rather than grouped into blocks on a single chain. In its original form, each new transaction had to approve two earlier transactions, so users who sent payments also helped confirm other people's. The design was meant to allow feeless, small payments between machines on the Internet of Things (IoT).

IOTA was founded in 2015 by David Sønstebø, Dominik Schiener, Sergey Ivancheglo and Serguei Popov, and Popov wrote the Tangle white paper, first released in 2016 and revised in later versions. The Tangle is the best-known example of a DAG-based distributed ledger technology (DLT), often compared with the hashgraph used by Hedera. IOTA's network has since changed substantially: in May 2025 its "IOTA Rebased" upgrade replaced the original Tangle ledger with a validator-run, delegated proof-of-stake design.

Imagine an office where, instead of a manager stamping every form, each person who files a new form must first check and initial two forms filed earlier by colleagues. The more people file forms, the more checking gets done, and nobody has to pay a clerk. A form that has been initialled, directly or indirectly, by many later forms is trusted as settled.

That was the Tangle's original idea. There were no miners collecting fees; every user did a small amount of checking as the price of joining. The busier the network, the faster transactions were expected to be confirmed.

Each transaction in the Tangle was a vertex in the graph. New, unapproved transactions at the edge of the graph were called tips. To issue a transaction, a user's software:

1. Chose two tips, using a random walk through the graph designed to favour tips on the main, honest part of the Tangle.
2. Checked that those two transactions did not conflict with each other or with their history.
3. Performed a small proof-of-work calculation, mainly to discourage spam, and then broadcast the new transaction pointing to the two tips it approved.

A transaction's confidence grew as more later transactions approved it, directly or indirectly. Because there were no blocks, no block rewards and no transaction fees, the design appealed to developers of micropayments and sensor-data markets, where a fee larger than the payment itself would make no sense.

In practice, a young DAG with little traffic is vulnerable: an attacker with modest computing power could outpace honest users and attempt double spending. To prevent this, IOTA ran a node called the Coordinator, operated by the IOTA Foundation, which issued signed "milestone" transactions; only transactions referenced by a milestone counted as confirmed. Critics argued this made the network centralized, since one organization's node had the final say.

The dependence became visible in February 2020, when the Foundation shut down the Coordinator after funds were stolen through a flaw in its Trinity wallet, halting confirmations until it was restarted on 10 March 2020. IOTA planned to remove the Coordinator in stages: the Chrysalis upgrade, live in April 2021, reworked the protocol, and a longer research effort called Coordicide aimed at a leaderless IOTA 2.0.

IOTA token holders approved a different route in December 2024. The IOTA Rebased upgrade, which began on 5 May 2025, moved balances from the previous Stardust network to a new network built on technology from Sui. According to the IOTA Foundation, it uses an object-based ledger, smart contracts written in the Move language, and a delegated proof-of-stake system in which validators must hold a minimum stake that token holders can help supply; it launched with 13 validators. The new design still uses a DAG internally to order messages between validators, but the Coordinator and the original user-approves-two-tips Tangle no longer secure the network.

The Tangle is an important experiment in scaling and feeless payments. Its history also shows the difficulty of reaching secure consensus without either mining power or stake: for most of its life the Tangle relied on a central Coordinator, and IOTA ultimately adopted a stake-based validator model similar to Proof of Stake (PoS) chains. For readers comparing ledger shapes, the Tangle sits alongside the conventional blockchain, the hashgraph and sharding as one of several approaches to handling more transactions on a shared ledger.

• IOTA documentation
• IOTA Foundation: IOTA Rebased technical view