diff --git a/Cslib.lean b/Cslib.lean
index a4d250711..c10c8d778 100644
--- a/Cslib.lean
+++ b/Cslib.lean
@@ -27,6 +27,7 @@ public import Cslib.Computability.Automata.NA.Sum
 public import Cslib.Computability.Automata.NA.ToDA
 public import Cslib.Computability.Automata.NA.Total
 public import Cslib.Computability.Automata.Transducers.Transducer
+public import Cslib.Computability.Automata.TwoNA.Basic
 public import Cslib.Computability.Distributed.FLP.Algorithm
 public import Cslib.Computability.Distributed.FLP.CanReachVia
 public import Cslib.Computability.Distributed.FLP.Consensus
diff --git a/Cslib/Computability/Automata/TwoNA/Basic.lean b/Cslib/Computability/Automata/TwoNA/Basic.lean
new file mode 100644
index 000000000..9b5e0c2b7
--- /dev/null
+++ b/Cslib/Computability/Automata/TwoNA/Basic.lean
@@ -0,0 +1,133 @@
+/-
+Copyright (c) 2026 Christian Reitwiessner. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Christian Reitwiessner
+-/
+
+module
+
+public import Cslib.Computability.Automata.Acceptors.Acceptor
+public import Mathlib.Data.List.Chain
+public import Mathlib.Data.Sign.Basic
+
+/-! # Nondeterministic Two-Way Automaton
+
+A Nondeterministic Two-Way Automaton (TwoNA) reads a finite input word on a tape and may move its
+input head in either direction. A transition reads the symbol under the head and, besides changing
+the state, moves the head one cell to the left, keeps it in place, or moves it one cell to the
+right.
+
+Once it leaves the word to the right, the automaton cannot move back to the word. A run is accepting
+if it ends in an accepting state with the head just past the end of the input.
+
+## Main definitions
+
+* `TwoNA`, the automaton itself: The transition relation and the sets of initial and accepting
+  states.
+* `TwoNACfg`, a configuration of a `TwoNA` running on a fixed input: a state together with a head
+  position.
+* `TwoNA.Step`, The single-step relation between configurations.
+* `TwoNA.Run`, a run of a `TwoNA` on a fixed input: a nonempty chain of configurations linked by
+  steps that starts in an initial configuration.
+
+## Implementation notes
+
+The definition of `TwoNA` is kept close to [Vardi][Vardi1989]'s, because the main point is to
+prove equivalence to `NA.FinAcc`. This means we do not allow the head to move off the input to the
+left, but also do not provide an end marker. Once the head moves off to the right, it cannot move
+back into the word.
+
+Runs are `List.IsChain` chains of configurations, anchored at an initial configuration.
+
+## References
+
+* [Moshe Y. Vardi, *A Note on the Reduction of Two-Way Automata to One-Way Automata*][Vardi1989]
+
+-/
+
+@[expose] public section
+
+namespace Cslib.Automata
+
+variable {State Symbol : Type*}
+
+/-- A nondeterministic two-way automaton: a transition relation that reads an input symbol and
+moves the input head, together with a set of initial and a set of accepting states. -/
+structure TwoNA (State Symbol : Type*) where
+  /-- The transition relation. `Tr q x m q'` means that, while reading the symbol `x`, the
+  automaton can move from state `q` to state `q'` and move its head according to `m`. -/
+  Tr (q : State) (x : Symbol) (m : SignType) (q' : State) : Prop
+  /-- The set of initial states of the automaton. -/
+  start : Set State
+  /-- The set of accepting states of the automaton. -/
+  accept : Set State
+
+/-- The configuration of a two-way nondeterministic automaton. -/
+@[ext]
+structure TwoNACfg (State Symbol : Type*) (input : List Symbol) where
+  /-- The state of the automaton. -/
+  state : State
+  /-- The input head position of the automaton: it can be on any symbol of the input or on the
+  position one step to the right of the input. -/
+  pos : Fin (input.length + 1)
+
+namespace TwoNA
+
+variable {a : TwoNA State Symbol} {input : List Symbol} {c c' : TwoNACfg State Symbol input}
+
+/-- A single step of `a` on `input`. It is possible only while the head is on an actual input
+symbol, and it is performed by a transition of `a` that reads the symbol under the head: such a
+transition determines the new state and a head movement `m`, by which the head position changes.
+A leftward move at position `0` would take the head off the input, so no such step exists. -/
+def Step (a : TwoNA State Symbol) (input : List Symbol)
+    (c c' : TwoNACfg State Symbol input) : Prop :=
+  ∃ m, ∃ _ : (c.pos : ℕ) < input.length,
+    a.Tr c.state input[c.pos] m c'.state ∧ (c'.pos : ℤ) = (c.pos : ℤ) + (m.cast : ℤ)
+
+/-- A run of `a` on `input`: a nonempty chain of configurations, each obtained from the previous
+one by a step, that starts in an initial state with the head on the first symbol of the input. -/
+structure Run (a : TwoNA State Symbol) (input : List Symbol) where
+  /-- The chain of configurations. -/
+  chain : List (TwoNACfg State Symbol input)
+  /-- Consecutive configurations are linked by a step. -/
+  isChain : chain.IsChain (a.Step input)
+  /-- There is at least one configuration. -/
+  ne : chain ≠ []
+  /-- The head starts on the first symbol of the input. -/
+  head_pos : (chain.head ne).pos = 0
+  /-- The run starts in an initial state. -/
+  head_mem_start : (chain.head ne).state ∈ a.start
+
+/-- The configuration a run starts in. -/
+def Run.head (r : a.Run input) : TwoNACfg State Symbol input := r.chain.head r.ne
+
+/-- The configuration a run ends in. -/
+def Run.last (r : a.Run input) : TwoNACfg State Symbol input := (r.chain).getLast r.ne
+
+/-- A run is accepting if it ends in an accepting state with the head just past the end of the
+input. -/
+def Run.IsAccepting (r : a.Run input) : Prop := r.last.state ∈ a.accept ∧ r.last.pos = Fin.last _
+
+/-- Extend a run by one more step. -/
+def Run.snoc (r : a.Run input) (c : TwoNACfg State Symbol input) (h : a.Step input r.last c) :
+    a.Run input where
+  chain := r.chain ++ [c]
+  isChain := r.isChain.append (List.isChain_singleton c)
+    (by simp_all [Run.last, List.getLast?_eq_some_getLast r.ne])
+  ne := by simp
+  head_pos := by simpa [List.head_append_of_ne_nil r.ne] using r.head_pos
+  head_mem_start := by simpa [List.head_append_of_ne_nil r.ne] using r.head_mem_start
+
+@[simp]
+theorem Run.last_snoc (r : a.Run input) (c : TwoNACfg State Symbol input)
+    (h : a.Step input r.last c) : (r.snoc c h).last = c := by
+  simp [Run.snoc, Run.last]
+
+end TwoNA
+
+/-- A `TwoNA` accepts an input if it has an accepting run on it. -/
+@[simp, scoped grind =]
+instance : Acceptor (TwoNA State Symbol) Symbol where
+  Accepts (a : TwoNA State Symbol) (xs : List Symbol) := ∃ r : a.Run xs, r.IsAccepting
+
+end Cslib.Automata
diff --git a/references.bib b/references.bib
index d2a7dfb13..48aa4fd76 100644
--- a/references.bib
+++ b/references.bib
@@ -551,3 +551,13 @@ @book{Papadimitriou94
   publisher={Addison-Wesley},
   address={Reading, Massachusetts}
 }
+
+@article{Vardi1989,
+  author       = {Moshe Y. Vardi},
+  title        = {A Note on the Reduction of Two-Way Automata to One-Way Automata},
+  journal      = {Information Processing Letters},
+  volume       = {30},
+  number       = {5},
+  pages        = {261--264},
+  year         = {1989}
+}