diff --git a/gtsam/base/types.cpp b/gtsam/base/types.cpp
index edc449b12..f96e037c1 100644
--- a/gtsam/base/types.cpp
+++ b/gtsam/base/types.cpp
@@ -64,4 +64,50 @@ std::string demangle(const char* name) {
   return demangled_name;
 }
 
+std::string GTDKeyFormatter(const Key &key) {
+  constexpr size_t kMax_uchar_ = std::numeric_limits<uint8_t>::max();
+  constexpr size_t key_bits = sizeof(gtsam::Key) * 8;
+  constexpr size_t ch1_bits = sizeof(uint8_t) * 8;
+  constexpr size_t ch2_bits = sizeof(uint8_t) * 8;
+  constexpr size_t link_bits = sizeof(uint8_t) * 8;
+  constexpr size_t joint_bits = sizeof(uint8_t) * 8;
+  constexpr size_t time_bits =
+      key_bits - ch1_bits - ch2_bits - link_bits - joint_bits;
+
+  constexpr gtsam::Key ch1_mask = gtsam::Key(kMax_uchar_)
+                                  << (key_bits - ch1_bits);
+  constexpr gtsam::Key ch2_mask = gtsam::Key(kMax_uchar_)
+                                  << (key_bits - ch1_bits - ch2_bits);
+  constexpr gtsam::Key link_mask = gtsam::Key(kMax_uchar_)
+                                   << (time_bits + joint_bits);
+  constexpr gtsam::Key joint_mask = gtsam::Key(kMax_uchar_) << time_bits;
+  constexpr gtsam::Key time_mask =
+      ~(ch1_mask | ch2_mask | link_mask | joint_mask);
+
+  uint8_t c1_, c2_, link_idx_, joint_idx_;
+  uint64_t t_;
+
+  c1_ = (uint8_t)((key & ch1_mask) >> (key_bits - ch1_bits));
+  c2_ = (uint8_t)((key & ch2_mask) >> (key_bits - ch1_bits - ch2_bits));
+  link_idx_ = (uint8_t)((key & link_mask) >> (time_bits + joint_bits));
+  joint_idx_ = (uint8_t)((key & joint_mask) >> time_bits);
+  t_ = key & time_mask;
+
+  std::string s = "";
+  if (c1_ != 0) {
+    s += c1_;
+  }
+  if (c2_ != 0) {
+    s += c2_;
+  }
+  if (link_idx_ != kMax_uchar_) {
+    s += "[" + std::to_string((int)(link_idx_)) + "]";
+  }
+  if (joint_idx_ != kMax_uchar_) {
+    s += "(" + std::to_string((int)(joint_idx_)) + ")";
+  }
+  s += std::to_string(t_);
+  return s;
+}
+
 } /* namespace gtsam */
diff --git a/gtsam/base/types.h b/gtsam/base/types.h
index a0d24f1a6..dcb389c9b 100644
--- a/gtsam/base/types.h
+++ b/gtsam/base/types.h
@@ -74,6 +74,8 @@ namespace gtsam {
   /// The index type for Eigen objects
   typedef ptrdiff_t DenseIndex;
 
+  std::string GTDKeyFormatter(const Key &key);
+
   /* ************************************************************************* */
   /**
    * Helper class that uses templates to select between two types based on
diff --git a/gtsam/hybrid/HybridGaussianFactorGraph.cpp b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
index 286129f8b..041603fbd 100644
--- a/gtsam/hybrid/HybridGaussianFactorGraph.cpp
+++ b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
@@ -55,52 +55,6 @@ namespace gtsam {
 
 template class EliminateableFactorGraph<HybridGaussianFactorGraph>;
 
-std::string GTDKeyFormatter(const Key &key) {
-  constexpr size_t kMax_uchar_ = std::numeric_limits<uint8_t>::max();
-  constexpr size_t key_bits = sizeof(gtsam::Key) * 8;
-  constexpr size_t ch1_bits = sizeof(uint8_t) * 8;
-  constexpr size_t ch2_bits = sizeof(uint8_t) * 8;
-  constexpr size_t link_bits = sizeof(uint8_t) * 8;
-  constexpr size_t joint_bits = sizeof(uint8_t) * 8;
-  constexpr size_t time_bits =
-      key_bits - ch1_bits - ch2_bits - link_bits - joint_bits;
-
-  constexpr gtsam::Key ch1_mask = gtsam::Key(kMax_uchar_)
-                                  << (key_bits - ch1_bits);
-  constexpr gtsam::Key ch2_mask = gtsam::Key(kMax_uchar_)
-                                  << (key_bits - ch1_bits - ch2_bits);
-  constexpr gtsam::Key link_mask = gtsam::Key(kMax_uchar_)
-                                   << (time_bits + joint_bits);
-  constexpr gtsam::Key joint_mask = gtsam::Key(kMax_uchar_) << time_bits;
-  constexpr gtsam::Key time_mask =
-      ~(ch1_mask | ch2_mask | link_mask | joint_mask);
-
-  uint8_t c1_, c2_, link_idx_, joint_idx_;
-  uint64_t t_;
-
-  c1_ = (uint8_t)((key & ch1_mask) >> (key_bits - ch1_bits));
-  c2_ = (uint8_t)((key & ch2_mask) >> (key_bits - ch1_bits - ch2_bits));
-  link_idx_ = (uint8_t)((key & link_mask) >> (time_bits + joint_bits));
-  joint_idx_ = (uint8_t)((key & joint_mask) >> time_bits);
-  t_ = key & time_mask;
-
-  std::string s = "";
-  if (c1_ != 0) {
-    s += c1_;
-  }
-  if (c2_ != 0) {
-    s += c2_;
-  }
-  if (link_idx_ != kMax_uchar_) {
-    s += "[" + std::to_string((int)(link_idx_)) + "]";
-  }
-  if (joint_idx_ != kMax_uchar_) {
-    s += "(" + std::to_string((int)(joint_idx_)) + ")";
-  }
-  s += std::to_string(t_);
-  return s;
-}
-
 /* ************************************************************************ */
 static GaussianMixtureFactor::Sum &addGaussian(
     GaussianMixtureFactor::Sum &sum, const GaussianFactor::shared_ptr &factor) {