diff --git a/builddefs/common_features.mk b/builddefs/common_features.mk
index 68f9a1dd08..3861a55c94 100644
--- a/builddefs/common_features.mk
+++ b/builddefs/common_features.mk
@@ -984,3 +984,9 @@ ifeq ($(strip $(UART_DRIVER_REQUIRED)), yes)
         QUANTUM_LIB_SRC += uart.c
     endif
 endif
+
+
+ifeq ($(strip $(ACHORDION_ENABLE)), yes)
+    OPT_DEFS += -DACHORDION_ENABLE -DPERMISSIVE_HOLD
+    SRC += $(QUANTUM_DIR)/process_keycode/process_achordion.c
+endif
diff --git a/quantum/keyboard.c b/quantum/keyboard.c
index df1dc1c3ee..bdb3217f5c 100644
--- a/quantum/keyboard.c
+++ b/quantum/keyboard.c
@@ -140,6 +140,9 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #ifdef OS_DETECTION_ENABLE
 #    include "os_detection.h"
 #endif
+#ifdef ACHORDION_ENABLE
+#    include "process_achordion.h"
+#endif
 
 static uint32_t last_input_modification_time = 0;
 uint32_t        last_input_activity_time(void) {
@@ -655,6 +658,10 @@ void quantum_task(void) {
 #ifdef SECURE_ENABLE
     secure_task();
 #endif
+
+#ifdef ACHORDION_ENABLE
+    achordion_task();
+#endif
 }
 
 /** \brief Main task that is repeatedly called as fast as possible. */
diff --git a/quantum/process_keycode/process_achordion.c b/quantum/process_keycode/process_achordion.c
new file mode 100644
index 0000000000..8e20eefe26
--- /dev/null
+++ b/quantum/process_keycode/process_achordion.c
@@ -0,0 +1,216 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/**
+ * @file achordion.c
+ * @brief Achordion implementation
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/achordion>
+ */
+
+#include "process_achordion.h"
+
+#if !defined(IS_QK_MOD_TAP)
+// Attempt to detect out-of-date QMK installation, which would fail with
+// implicit-function-declaration errors in the code below.
+#    error "achordion: QMK version is too old to build. Please update QMK."
+#else
+
+// Copy of the `record` and `keycode` args for the current active tap-hold key.
+static keyrecord_t tap_hold_record;
+static uint16_t    tap_hold_keycode = KC_NO;
+// Timeout timer. When it expires, the key is considered held.
+static uint16_t hold_timer = 0;
+// Eagerly applied mods, if any.
+static uint8_t eager_mods = 0;
+
+// Achordion's current state.
+enum {
+    // A tap-hold key is pressed, but hasn't yet been settled as tapped or held.
+    STATE_UNSETTLED,
+    // Achordion is inactive.
+    STATE_RELEASED,
+    // Active tap-hold key has been settled as tapped.
+    STATE_TAPPING,
+    // Active tap-hold key has been settled as held.
+    STATE_HOLDING,
+    // This state is set while calling `process_record()`, which will recursively
+    // call `process_achordion()`. This state is checked so that we don't process
+    // events generated by Achordion and potentially create an infinite loop.
+    STATE_RECURSING,
+};
+static uint8_t achordion_state = STATE_RELEASED;
+
+// Calls `process_record()` with state set to RECURSING.
+static void recursively_process_record(keyrecord_t* record, uint8_t state) {
+    achordion_state = STATE_RECURSING;
+    process_record(record);
+    achordion_state = state;
+}
+
+// Clears eagerly-applied mods.
+static void clear_eager_mods(void) {
+    unregister_mods(eager_mods);
+    eager_mods = 0;
+}
+
+// Sends hold press event and settles the active tap-hold key as held.
+static void settle_as_hold(void) {
+    clear_eager_mods();
+    // Create hold press event.
+    recursively_process_record(&tap_hold_record, STATE_HOLDING);
+}
+
+bool process_achordion(uint16_t keycode, keyrecord_t* record) {
+    // Don't process events that Achordion generated.
+    if (achordion_state == STATE_RECURSING) {
+        return true;
+    }
+
+    // Determine whether the current event is for a mod-tap or layer-tap key.
+    const bool is_mt       = IS_QK_MOD_TAP(keycode);
+    const bool is_tap_hold = is_mt || IS_QK_LAYER_TAP(keycode);
+    // Check that this is a normal key event, don't act on combos.
+#    ifdef IS_KEYEVENT
+    const bool is_key_event = IS_KEYEVENT(record->event);
+#    else
+    const bool is_key_event = (record->event.key.row < 254 && record->event.key.col < 254);
+#    endif
+
+    if (achordion_state == STATE_RELEASED) {
+        if (is_tap_hold && record->tap.count == 0 && record->event.pressed && is_key_event) {
+            // A tap-hold key is pressed and considered by QMK as "held".
+            const uint16_t timeout = achordion_timeout(keycode);
+            if (timeout > 0) {
+                achordion_state = STATE_UNSETTLED;
+                // Save info about this key.
+                tap_hold_keycode = keycode;
+                tap_hold_record  = *record;
+                hold_timer       = record->event.time + timeout;
+
+                if (is_mt) { // Apply mods immediately if they are "eager."
+                    uint8_t mod = mod_config(QK_MOD_TAP_GET_MODS(tap_hold_keycode));
+                    if (achordion_eager_mod(mod)) {
+                        eager_mods = ((mod & 0x10) == 0) ? mod : (mod << 4);
+                        register_mods(eager_mods);
+                    }
+                }
+
+                dprintf("Achordion: Key 0x%04X pressed.%s\n", keycode, eager_mods ? " Set eager mods." : "");
+                return false; // Skip default handling.
+            }
+        }
+
+        return true; // Otherwise, continue with default handling.
+    }
+
+    if (keycode == tap_hold_keycode && !record->event.pressed) {
+        // The active tap-hold key is being released.
+        if (achordion_state == STATE_HOLDING) {
+            dprintln("Achordion: Key released. Plumbing hold release.");
+            tap_hold_record.event.pressed = false;
+            // Plumb hold release event.
+            recursively_process_record(&tap_hold_record, STATE_RELEASED);
+        } else {
+            dprintf("Achordion: Key released.%s\n", eager_mods ? " Clearing eager mods." : "");
+            if (is_mt) {
+                clear_eager_mods();
+            }
+        }
+
+        achordion_state = STATE_RELEASED;
+        return false;
+    }
+
+    if (achordion_state == STATE_UNSETTLED && record->event.pressed) {
+        // Press event occurred on a key other than the active tap-hold key.
+
+        // If the other key is *also* a tap-hold key and considered by QMK to be
+        // held, then we settle the active key as held. This way, things like
+        // chording multiple home row modifiers will work, but let's our logic
+        // consider simply a single tap-hold key as "active" at a time.
+        //
+        // Otherwise, we call `achordion_chord()` to determine whether to settle the
+        // tap-hold key as tapped vs. held. We implement the tap or hold by plumbing
+        // events back into the handling pipeline so that QMK features and other
+        // user code can see them. This is done by calling `process_record()`, which
+        // in turn calls most handlers including `process_record_user()`.
+        if (!is_key_event || (is_tap_hold && record->tap.count == 0) || achordion_chord(tap_hold_keycode, &tap_hold_record, keycode, record)) {
+            dprintln("Achordion: Plumbing hold press.");
+            settle_as_hold();
+        } else {
+            clear_eager_mods(); // Clear in case eager mods were set.
+
+            dprintln("Achordion: Plumbing tap press.");
+            tap_hold_record.tap.count       = 1; // Revise event as a tap.
+            tap_hold_record.tap.interrupted = true;
+            // Plumb tap press event.
+            recursively_process_record(&tap_hold_record, STATE_TAPPING);
+
+            send_keyboard_report();
+#    if TAP_CODE_DELAY > 0
+            wait_ms(TAP_CODE_DELAY);
+#    endif // TAP_CODE_DELAY > 0
+
+            dprintln("Achordion: Plumbing tap release.");
+            tap_hold_record.event.pressed = false;
+            // Plumb tap release event.
+            recursively_process_record(&tap_hold_record, STATE_TAPPING);
+        }
+
+        recursively_process_record(record, achordion_state); // Re-process event.
+        return false;                                        // Block the original event.
+    }
+
+    return true;
+}
+
+void achordion_task(void) {
+    if (achordion_state == STATE_UNSETTLED && timer_expired(timer_read(), hold_timer)) {
+        dprintln("Achordion: Timeout. Plumbing hold press.");
+        settle_as_hold(); // Timeout expired, settle the key as held.
+    }
+}
+
+// Returns true if `pos` on the left hand of the keyboard, false if right.
+static bool on_left_hand(keypos_t pos) {
+#    ifdef SPLIT_KEYBOARD
+    return pos.row < MATRIX_ROWS / 2;
+#    else
+    return (MATRIX_COLS > MATRIX_ROWS) ? pos.col < MATRIX_COLS / 2 : pos.row < MATRIX_ROWS / 2;
+#    endif
+}
+
+bool achordion_opposite_hands(const keyrecord_t* tap_hold_record, const keyrecord_t* other_record) {
+    return on_left_hand(tap_hold_record->event.key) != on_left_hand(other_record->event.key);
+}
+
+// By default, use the BILATERAL_COMBINATIONS rule to consider the tap-hold key
+// "held" only when it and the other key are on opposite hands.
+__attribute__((weak)) bool achordion_chord(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record, uint16_t other_keycode, keyrecord_t* other_record) {
+    return achordion_opposite_hands(tap_hold_record, other_record);
+}
+
+// By default, the timeout is 1000 ms for all keys.
+__attribute__((weak)) uint16_t achordion_timeout(uint16_t tap_hold_keycode) {
+    return 1000;
+}
+
+// By default, Shift and Ctrl mods are eager, and Alt and GUI are not.
+__attribute__((weak)) bool achordion_eager_mod(uint8_t mod) {
+    return (mod & (MOD_LALT | MOD_LGUI)) == 0;
+}
+
+#endif // version check
diff --git a/quantum/process_keycode/process_achordion.h b/quantum/process_keycode/process_achordion.h
new file mode 100644
index 0000000000..0c1bc69093
--- /dev/null
+++ b/quantum/process_keycode/process_achordion.h
@@ -0,0 +1,162 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/**
+ * @file achordion.h
+ * @brief Achordion: Customizing the tap-hold decision.
+ *
+ * Overview
+ * --------
+ *
+ * This library customizes when tap-hold keys are considered held vs. tapped
+ * based on the next pressed key, like Manna Harbour's Bilateral Combinations or
+ * ZMK's positional hold. The library works on top of QMK's existing tap-hold
+ * implementation. You define mod-tap and layer-tap keys as usual and use
+ * Achordion to fine-tune the behavior.
+ *
+ * When QMK settles a tap-hold key as held, Achordion intercepts the event.
+ * Achordion then revises the event as a tap or passes it along as a hold:
+ *
+ *  * Chord condition: On the next key press, a customizable `achordion_chord()`
+ *    function is called, which takes the tap-hold key and the next key pressed
+ *    as args. When the function returns true, the tap-hold key is settled as
+ *    held, and otherwise as tapped.
+ *
+ *  * Timeout: If no other key press occurs within a timeout, the tap-hold key
+ *    is settled as held. This is customizable with `achordion_timeout()`.
+ *
+ * Achordion only changes the behavior when QMK considered the key held. It
+ * changes some would-be holds to taps, but no taps to holds.
+ *
+ * @note Some QMK features handle events before the point where Achordion can
+ * intercept them, particularly: Combos, Key Lock, and Dynamic Macros. It's
+ * still possible to use these features and Achordion in your keymap, but beware
+ * they might behave poorly when used simultaneously with tap-hold keys.
+ *
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/achordion>
+ */
+
+#pragma once
+
+#include "quantum.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Handler function for Achordion.
+ *
+ * Call this function from `process_record_user()` as
+ *
+ *     #include "features/achordion.h"
+ *
+ *     bool process_record_user(uint16_t keycode, keyrecord_t* record) {
+ *       if (!process_achordion(keycode, record)) { return false; }
+ *       // Your macros...
+ *       return true;
+ *     }
+ */
+bool process_achordion(uint16_t keycode, keyrecord_t* record);
+
+/**
+ * Matrix task function for Achordion.
+ *
+ * Call this function from `matrix_scan_user()` as
+ *
+ *     void matrix_scan_user(void) {
+ *       achordion_task();
+ *     }
+ */
+void achordion_task(void);
+
+/**
+ * Optional callback to customize which key chords are considered "held".
+ *
+ * In your keymap.c, define the callback
+ *
+ *     bool achordion_chord(uint16_t tap_hold_keycode,
+ *                          keyrecord_t* tap_hold_record,
+ *                          uint16_t other_keycode,
+ *                          keyrecord_t* other_record) {
+ *        // Conditions...
+ *     }
+ *
+ * This callback is called if while `tap_hold_keycode` is pressed,
+ * `other_keycode` is pressed. Return true if the tap-hold key should be
+ * considered held, or false to consider it tapped.
+ *
+ * @param tap_hold_keycode Keycode of the tap-hold key.
+ * @param tap_hold_record keyrecord_t from the tap-hold press event.
+ * @param other_keycode Keycode of the other key.
+ * @param other_record keyrecord_t from the other key's press event.
+ * @return True if the tap-hold key should be considered held.
+ */
+bool achordion_chord(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record, uint16_t other_keycode, keyrecord_t* other_record);
+
+/**
+ * Optional callback to define a timeout duration per keycode.
+ *
+ * In your keymap.c, define the callback
+ *
+ *     uint16_t achordion_timeout(uint16_t tap_hold_keycode) {
+ *       // ...
+ *     }
+ *
+ * The callback determines Achordion's timeout duration for `tap_hold_keycode`
+ * in units of milliseconds. The timeout be in the range 0 to 32767 ms (upper
+ * bound is due to 16-bit timer limitations). Use a timeout of 0 to bypass
+ * Achordion.
+ *
+ * @param tap_hold_keycode Keycode of the tap-hold key.
+ * @return Timeout duration in milliseconds in the range 0 to 32767.
+ */
+uint16_t achordion_timeout(uint16_t tap_hold_keycode);
+
+/**
+ * Optional callback defining which mods are "eagerly" applied.
+ *
+ * This callback defines  which mods are "eagerly" applied while a mod-tap
+ * key is still being settled. This is helpful to reduce delay particularly when
+ * using mod-tap keys with an external mouse.
+ *
+ * Define this callback in your keymap.c. The default callback is eager for
+ * Shift and Ctrl, and not for Alt and GUI:
+ *
+ *     bool achordion_eager_mod(uint8_t mod) {
+ *       return (mod & (MOD_LALT | MOD_LGUI)) == 0;
+ *     }
+ *
+ * @note `mod` should be compared with `MOD_` prefixed codes, not `KC_` codes,
+ * described at <https://docs.qmk.fm/#/mod_tap>.
+ *
+ * @param mod Modifier `MOD_` code.
+ * @return True if the modifier should be eagerly applied.
+ */
+bool achordion_eager_mod(uint8_t mod);
+
+/**
+ * Returns true if the args come from keys on opposite hands.
+ *
+ * @param tap_hold_record keyrecord_t from the tap-hold key's event.
+ * @param other_record keyrecord_t from the other key's event.
+ * @return True if the keys are on opposite hands.
+ */
+bool achordion_opposite_hands(const keyrecord_t* tap_hold_record, const keyrecord_t* other_record);
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/quantum/quantum.c b/quantum/quantum.c
index 011f9d73e4..7c267f4a17 100644
--- a/quantum/quantum.c
+++ b/quantum/quantum.c
@@ -323,6 +323,9 @@ bool process_record_quantum(keyrecord_t *record) {
 #endif
 #if defined(POINTING_DEVICE_ENABLE) && defined(POINTING_DEVICE_AUTO_MOUSE_ENABLE)
             process_auto_mouse(keycode, record) &&
+#endif
+#ifdef ACHORDION_ENABLE
+            process_achordion(keycode, record) &&
 #endif
             process_record_kb(keycode, record) &&
 #if defined(SECURE_ENABLE)
diff --git a/quantum/quantum.h b/quantum/quantum.h
index 5446ab1ad7..9c978f3c19 100644
--- a/quantum/quantum.h
+++ b/quantum/quantum.h
@@ -65,6 +65,9 @@
 #ifdef DEFERRED_EXEC_ENABLE
 #    include "deferred_exec.h"
 #endif
+#ifdef ACHORDION_ENABLE
+#    include "process_achordion.h"
+#endif
 
 extern layer_state_t default_layer_state;