diff --git a/src/factor/mod.rs b/src/factor/mod.rs
index 3580fa4..b0ce1b9 100644
--- a/src/factor/mod.rs
+++ b/src/factor/mod.rs
@@ -47,6 +47,56 @@ impl VarStore {
     }
 }
 
+/// A factor in the EP graph.
+///
+/// Factors hold their own outgoing messages and propagate them by reading
+/// connected variable marginals from a `VarStore` and writing back updated
+/// marginals.
+pub(crate) trait Factor {
+    /// Update outgoing messages and write back to the var store.
+    ///
+    /// Returns the max delta `(|Δmu|, |Δsigma|)` across writes this
+    /// propagation. Used by the `Schedule` to detect convergence.
+    fn propagate(&mut self, vars: &mut VarStore) -> (f64, f64);
+
+    /// Optional log-evidence contribution. Default 0.0 (no contribution).
+    fn log_evidence(&self, _vars: &VarStore) -> f64 {
+        0.0
+    }
+}
+
+/// Enum dispatcher for the built-in factor types.
+///
+/// Using an enum instead of `Box<dyn Factor>` keeps factor data inline and
+/// avoids virtual-call overhead in the hot inference loop.
+#[derive(Debug)]
+pub(crate) enum BuiltinFactor {
+    TeamSum(team_sum::TeamSumFactor),
+    RankDiff(rank_diff::RankDiffFactor),
+    Trunc(trunc::TruncFactor),
+}
+
+impl Factor for BuiltinFactor {
+    fn propagate(&mut self, vars: &mut VarStore) -> (f64, f64) {
+        match self {
+            Self::TeamSum(f) => f.propagate(vars),
+            Self::RankDiff(f) => f.propagate(vars),
+            Self::Trunc(f) => f.propagate(vars),
+        }
+    }
+
+    fn log_evidence(&self, vars: &VarStore) -> f64 {
+        match self {
+            Self::Trunc(f) => f.log_evidence(vars),
+            _ => 0.0,
+        }
+    }
+}
+
+pub(crate) mod rank_diff;
+pub(crate) mod team_sum;
+pub(crate) mod trunc;
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/src/factor/rank_diff.rs b/src/factor/rank_diff.rs
new file mode 100644
index 0000000..9ecf995
--- /dev/null
+++ b/src/factor/rank_diff.rs
@@ -0,0 +1,14 @@
+use crate::factor::{Factor, VarId, VarStore};
+
+#[derive(Debug)]
+pub(crate) struct RankDiffFactor {
+    pub(crate) team_a: VarId,
+    pub(crate) team_b: VarId,
+    pub(crate) diff: VarId,
+}
+
+impl Factor for RankDiffFactor {
+    fn propagate(&mut self, _vars: &mut VarStore) -> (f64, f64) {
+        unimplemented!("RankDiffFactor stub — implemented in Task 5")
+    }
+}
diff --git a/src/factor/team_sum.rs b/src/factor/team_sum.rs
new file mode 100644
index 0000000..1619ce0
--- /dev/null
+++ b/src/factor/team_sum.rs
@@ -0,0 +1,16 @@
+use crate::{
+    factor::{Factor, VarId, VarStore},
+    gaussian::Gaussian,
+};
+
+#[derive(Debug)]
+pub(crate) struct TeamSumFactor {
+    pub(crate) inputs: Vec<(Gaussian, f64)>,
+    pub(crate) out: VarId,
+}
+
+impl Factor for TeamSumFactor {
+    fn propagate(&mut self, _vars: &mut VarStore) -> (f64, f64) {
+        unimplemented!("TeamSumFactor stub — implemented in Task 4")
+    }
+}
diff --git a/src/factor/trunc.rs b/src/factor/trunc.rs
new file mode 100644
index 0000000..f5b6dfe
--- /dev/null
+++ b/src/factor/trunc.rs
@@ -0,0 +1,32 @@
+use crate::{
+    N_INF,
+    factor::{Factor, VarId, VarStore},
+    gaussian::Gaussian,
+};
+
+#[derive(Debug)]
+pub(crate) struct TruncFactor {
+    pub(crate) diff: VarId,
+    pub(crate) margin: f64,
+    pub(crate) tie: bool,
+    pub(crate) msg: Gaussian,
+    pub(crate) evidence_cached: Option<f64>,
+}
+
+impl TruncFactor {
+    pub(crate) fn new(diff: VarId, margin: f64, tie: bool) -> Self {
+        Self {
+            diff,
+            margin,
+            tie,
+            msg: N_INF,
+            evidence_cached: None,
+        }
+    }
+}
+
+impl Factor for TruncFactor {
+    fn propagate(&mut self, _vars: &mut VarStore) -> (f64, f64) {
+        unimplemented!("TruncFactor stub — implemented in Task 6")
+    }
+}