The Tangle was IOTA’s original ledger: a directed acyclic graph (DAG) of transactions rather than a chain of blocks. It defined the network from launch until the IOTA Rebased upgrade in May 2025 retired it. Understanding it still matters, because most third-party material describes IOTA as if the Tangle were the live system.

What the Tangle Is#

In a blockchain, miners or validators batch transactions into blocks and link the blocks in a line. The Tangle had no blocks and no dedicated block producers. Each new transaction directly approved two previous transactions by referencing them, forming a growing mesh:

  • A transaction with no approvals yet is a tip.
  • Issuing a transaction required selecting two tips, verifying they did not conflict, and doing a small proof-of-work to deter spam.
  • Confirmation strength was measured by cumulative weight — how many later transactions indirectly approved a given one.

Because issuing a transaction also did the work of validating two others, throughput was meant to increase with usage rather than contend for scarce block space.

Feeless by Design#

With no miners competing for block rewards, there was no fee market. The sender’s proof-of-work substituted for a fee. This was the entire point: IOTA targeted the machine economy, where billions of micropayments between devices would be uneconomical if each carried a gas cost. Feelessness, not smart-contract expressiveness, was the original value proposition.

The Coordinator#

The Tangle had a well-known caveat. While the network was small, an attacker with modest hashing power could outpace honest tip approval and rewrite history (a “parasite chain” attack). IOTA’s mitigation was the Coordinator: a node operated by the IOTA Foundation that periodically issued signed milestones. A transaction was considered confirmed only once referenced by a milestone.

This made the Coordinator a single point of trust and control — it could, in principle, censor or halt the network — and it drew sustained criticism that IOTA was not genuinely decentralised. Removing it was a years-long research programme branded Coordicide (and later IOTA 2.0).

Why It Was Replaced#

Coordicide never reached a production mainnet under the Tangle model. Meanwhile the ecosystem’s demands shifted toward expressive smart contracts, deterministic finality, and competitive throughput — areas where the account/UTXO-free DAG was awkward to extend. Rather than ship Coordicide, the Foundation pivoted: IOTA Rebased replaced the Tangle ledger entirely with a Move-based object ledger and delegated-proof-of-stake consensus.

A subtle point often muddled in coverage: Rebased did not abandon DAGs. Its Mysticeti consensus is itself DAG-structured. What was retired is the Tangle as the ledger and validation model — the feeless, Coordinator-gated, tip-approval design — not the use of a DAG anywhere in the stack.

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