diff --git a/layout.c b/layout.c
index 3639e18..c80f7d2 100644
--- a/layout.c
+++ b/layout.c
@@ -185,3 +185,42 @@ const int * const bb_layout_get_scancodes(lv_obj_t *keyboard, uint16_t btn_id, i
     const int index = sq2lv_layouts[current_layout_id].layers[layer_index].scancode_idxs[btn_id];
     return &(sq2lv_layouts[current_layout_id].layers[layer_index].scancodes[index]);
 }
+
+bool bb_is_modifier(lv_obj_t *keyboard, uint16_t btn_id) {
+    if (current_layout_id < 0 || current_layout_id >= sq2lv_num_layouts) {
+        return false;
+    }
+
+    int layer_index = get_layer_index(keyboard);
+    if (layer_index < 0 || layer_index >= sq2lv_layouts[current_layout_id].num_layers) {
+        return false;
+    }
+
+    for (int i = 0; i < sq2lv_layouts[current_layout_id].layers[layer_index].num_modifiers; ++i) {
+        if (sq2lv_layouts[current_layout_id].layers[layer_index].modifier_idxs[i] == btn_id) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+const int * const bb_get_modifier_indexes(lv_obj_t *keyboard, int *num_modifiers) {
+    if (current_layout_id < 0 || current_layout_id >= sq2lv_num_layouts) {
+        *num_modifiers = 0;
+        return NULL;
+    }
+
+    int layer_index = get_layer_index(keyboard);
+    if (layer_index < 0 || layer_index >= sq2lv_layouts[current_layout_id].num_layers) {
+        *num_modifiers = 0;
+        return NULL;
+    }
+
+    *num_modifiers = sq2lv_layouts[current_layout_id].layers[layer_index].num_modifiers;
+    if (*num_modifiers == 0) {
+        return NULL;
+    }
+
+    return &(sq2lv_layouts[current_layout_id].layers[layer_index].modifier_idxs[0]);
+}
diff --git a/layout.h b/layout.h
index 7d53a2d..ae5b765 100644
--- a/layout.h
+++ b/layout.h
@@ -32,7 +32,7 @@
 void bb_layout_switch_layout(lv_obj_t *keyboard, sq2lv_layout_id_t layout_id);
 
 /**
- * Attempt to perform a layer switch after pressing a key.
+ * Attempt to perform a layer switch after pressing a key in the current layer.
  *
  * @param keyboard keyboard widget
  * @param btn_id button index corresponding to the pressed key
@@ -41,8 +41,8 @@ void bb_layout_switch_layout(lv_obj_t *keyboard, sq2lv_layout_id_t layout_id);
 bool bb_layout_switch_layer(lv_obj_t *keyboard, uint16_t btn_id);
 
 /**
- * Get scancodes associated with a key.
- * 
+ * Get scancodes associated with a key in the current layer.
+ *
  * @param keyboard keyboard widget
  * @param btn_id button index corresponding to the key
  * @param num_scancodes pointer to an integer into which the number of scancodes will be written
@@ -50,4 +50,22 @@ bool bb_layout_switch_layer(lv_obj_t *keyboard, uint16_t btn_id);
  */
 const int * const bb_layout_get_scancodes(lv_obj_t *keyboard, uint16_t btn_id, int *num_scancodes);
 
+/**
+ * Check if a key is a modifier in the current layer.
+ *
+ * @param keyboard keyboard widget
+ * @param btn_id button index corresponding to the key
+ * @return true if the key is a modifier, false otherwise
+ */
+bool bb_is_modifier(lv_obj_t *keyboard, uint16_t btn_id);
+
+/**
+ * Get the button indexes for all modifier keys in the current layer.
+ *
+ * @param keyboard keyboard widget
+ * @param num_modifiers pointer to an integer into which the number of modifiers will be written
+ * @return pointer to the array of button indexes corresponding to modifier keys
+ */
+const int * const bb_get_modifier_indexes(lv_obj_t *keyboard, int *num_modifiers);
+
 #endif /* BB_LAYOUT_H */
diff --git a/main.c b/main.c
index 762dbf0..a85b817 100644
--- a/main.c
+++ b/main.c
@@ -69,6 +69,16 @@ static void set_theme(bool is_dark);
  */
 static void keyboard_value_changed_cb(lv_event_t *event);
 
+/**
+ * Emit key down and up events for a key.
+ *
+ * @param keyboard keyboard widget
+ * @param btn_id button index corresponding to the key
+ * @param key_down true if a key down event should be emitted
+ * @param key_up true if a key up event should be emitted
+ */
+static void emit_key_events(lv_obj_t *keyboard, uint16_t btn_id, bool key_down, bool key_up);
+
 
 /**
  * Static functions
@@ -90,17 +100,47 @@ static void keyboard_value_changed_cb(lv_event_t *event) {
         return;
     }
 
+    /* Note that the LV_BTNMATRIX_CTRL_CHECKED logic is inverted because LV_KEYBOARD_CTRL_BTN_FLAGS already
+     * contains LV_BTNMATRIX_CTRL_CHECKED. As a result, pressing e.g. CTRL will _un_check the key. To account
+     * for this, we invert the meaning of "checked" below. */
+
+    bool is_modifier = bb_is_modifier(keyboard, btn_id);
+    bool is_checked = !lv_btnmatrix_has_btn_ctrl(keyboard, btn_id, LV_BTNMATRIX_CTRL_CHECKED);
+
+    /* Emit key events. Suppress key up events for modifiers unless they were unchecked. For checked modifiers
+     * the key up events are sent with the next non-modifier key press. */
+    emit_key_events(keyboard, btn_id, true, !is_modifier || !is_checked);
+
+    /* Pop any previously checked modifiers when a non-modifier key was pressed */
+    if (!is_modifier) {
+        int num_modifiers = 0;
+        int *modifier_idxs = bb_get_modifier_indexes(keyboard, &num_modifiers);
+
+        for (int i = 0; i < num_modifiers; ++i) {
+            if (!lv_btnmatrix_has_btn_ctrl(keyboard, modifier_idxs[i], LV_BTNMATRIX_CTRL_CHECKED)) {
+                emit_key_events(keyboard, modifier_idxs[i], false, true);
+                lv_btnmatrix_set_btn_ctrl(keyboard, modifier_idxs[i], LV_BTNMATRIX_CTRL_CHECKED);
+            }
+        }
+    }
+}
+
+static void emit_key_events(lv_obj_t *keyboard, uint16_t btn_id, bool key_down, bool key_up) {
     int num_scancodes = 0;
     int *scancodes = bb_layout_get_scancodes(keyboard, btn_id, &num_scancodes);
 
-    /* Emit key downs in forward order */
-    for (int i = 0; i < num_scancodes; ++i) {
-        bb_uinput_device_emit_key_down(scancodes[i]);
+    if (key_down) {
+        /* Emit key down events in forward order */
+        for (int i = 0; i < num_scancodes; ++i) {
+            bb_uinput_device_emit_key_down(scancodes[i]);
+        }
     }
 
-    /* Emit key ups in backward order */
-    for (int i = num_scancodes - 1; i >= 0; --i) {
-        bb_uinput_device_emit_key_up(scancodes[i]);
+    if (key_up) {
+        /* Emit key up events in backward order */
+        for (int i = num_scancodes - 1; i >= 0; --i) {
+            bb_uinput_device_emit_key_up(scancodes[i]);
+        }
     }
 }
 
diff --git a/sq2lv_layouts.c b/sq2lv_layouts.c
index bb290cf..73c8e42 100644
--- a/sq2lv_layouts.c
+++ b/sq2lv_layouts.c
@@ -24,13 +24,19 @@ static const char * const keycaps_lower_terminal_us[] = { \
 };
 
 static const lv_btnmatrix_ctrl_t attributes_lower_terminal_us[] = { \
-    LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
+    LV_KEYBOARD_CTRL_BTN_FLAGS | LV_BTNMATRIX_CTRL_CHECKABLE | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | LV_BTNMATRIX_CTRL_CHECKABLE | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
     2, 2, 2, 2, 2, 2, 2, 2, 2, 2, \
     2, 2, 2, 2, 2, 2, 2, 2, 2, \
     LV_KEYBOARD_CTRL_BTN_FLAGS | 3, 2, 2, 2, 2, 2, 2, 2, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
     LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 7, LV_KEYBOARD_CTRL_BTN_FLAGS | 3 \
 };
 
+static const int num_modifiers_lower_terminal_us = 2;
+
+static const int modifier_idxs_lower_terminal_us[] = { \
+    0, 1 \
+};
+
 static const int num_switchers_lower_terminal_us = 2;
 
 static const int switcher_idxs_lower_terminal_us[] = { \
@@ -80,13 +86,19 @@ static const char * const keycaps_upper_terminal_us[] = { \
 };
 
 static const lv_btnmatrix_ctrl_t attributes_upper_terminal_us[] = { \
-    LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
+    LV_KEYBOARD_CTRL_BTN_FLAGS | LV_BTNMATRIX_CTRL_CHECKABLE | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | LV_BTNMATRIX_CTRL_CHECKABLE | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
     2, 2, 2, 2, 2, 2, 2, 2, 2, 2, \
     2, 2, 2, 2, 2, 2, 2, 2, 2, \
     LV_KEYBOARD_CTRL_BTN_FLAGS | 3, 2, 2, 2, 2, 2, 2, 2, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
     LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 7, LV_KEYBOARD_CTRL_BTN_FLAGS | 3 \
 };
 
+static const int num_modifiers_upper_terminal_us = 2;
+
+static const int modifier_idxs_upper_terminal_us[] = { \
+    0, 1 \
+};
+
 static const int num_switchers_upper_terminal_us = 2;
 
 static const int switcher_idxs_upper_terminal_us[] = { \
@@ -136,13 +148,19 @@ static const char * const keycaps_numbers_terminal_us[] = { \
 };
 
 static const lv_btnmatrix_ctrl_t attributes_numbers_terminal_us[] = { \
-    LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
+    LV_KEYBOARD_CTRL_BTN_FLAGS | LV_BTNMATRIX_CTRL_CHECKABLE | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | LV_BTNMATRIX_CTRL_CHECKABLE | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
     2, 2, 2, 2, 2, 2, 2, 2, 2, 2, \
     2, 2, 2, 2, 2, 2, 2, 2, 2, 2, \
     2, 2, 2, 2, 2, 2, 2, LV_KEYBOARD_CTRL_BTN_FLAGS | 3, \
     LV_KEYBOARD_CTRL_BTN_FLAGS | 3, LV_KEYBOARD_CTRL_BTN_FLAGS | 7, 2, LV_KEYBOARD_CTRL_BTN_FLAGS | 3 \
 };
 
+static const int num_modifiers_numbers_terminal_us = 2;
+
+static const int modifier_idxs_numbers_terminal_us[] = { \
+    0, 1 \
+};
+
 static const int num_switchers_numbers_terminal_us = 1;
 
 static const int switcher_idxs_numbers_terminal_us[] = { \
@@ -188,6 +206,8 @@ static const sq2lv_layer_t layers_terminal_us[] = {
         .num_keys = num_keys_lower_terminal_us,
         .keycaps = keycaps_lower_terminal_us,
         .attributes = attributes_lower_terminal_us,
+        .num_modifiers = num_modifiers_lower_terminal_us,
+        .modifier_idxs = modifier_idxs_lower_terminal_us,
         .num_switchers = num_switchers_lower_terminal_us,
         .switcher_idxs = switcher_idxs_lower_terminal_us,
         .switcher_dests = switcher_dests_lower_terminal_us,
@@ -200,6 +220,8 @@ static const sq2lv_layer_t layers_terminal_us[] = {
         .num_keys = num_keys_upper_terminal_us,
         .keycaps = keycaps_upper_terminal_us,
         .attributes = attributes_upper_terminal_us,
+        .num_modifiers = num_modifiers_upper_terminal_us,
+        .modifier_idxs = modifier_idxs_upper_terminal_us,
         .num_switchers = num_switchers_upper_terminal_us,
         .switcher_idxs = switcher_idxs_upper_terminal_us,
         .switcher_dests = switcher_dests_upper_terminal_us,
@@ -212,6 +234,8 @@ static const sq2lv_layer_t layers_terminal_us[] = {
         .num_keys = num_keys_numbers_terminal_us,
         .keycaps = keycaps_numbers_terminal_us,
         .attributes = attributes_numbers_terminal_us,
+        .num_modifiers = num_modifiers_numbers_terminal_us,
+        .modifier_idxs = modifier_idxs_numbers_terminal_us,
         .num_switchers = num_switchers_numbers_terminal_us,
         .switcher_idxs = switcher_idxs_numbers_terminal_us,
         .switcher_dests = switcher_dests_numbers_terminal_us,
diff --git a/sq2lv_layouts.h b/sq2lv_layouts.h
index 674274b..e8cff27 100644
--- a/sq2lv_layouts.h
+++ b/sq2lv_layouts.h
@@ -20,6 +20,10 @@ typedef struct {
     const char ** const keycaps;
     /* Button matrix attributes */
     const lv_btnmatrix_ctrl_t * const attributes;
+    /* Number of modifier keys */
+    const int num_modifiers;
+    /* Button indexes of modifier keys */
+    const int * const modifier_idxs;
     /* Number of buttons that trigger a layer switch */
     const int num_switchers;
     /* Button indexes that trigger a layer switch */
diff --git a/squeek2lvgl b/squeek2lvgl
index 5f10b71..c0e99da 160000
--- a/squeek2lvgl
+++ b/squeek2lvgl
@@ -1 +1 @@
-Subproject commit 5f10b71d79e33b86c157c21b25575c46ca25c254
+Subproject commit c0e99da8f58e525d87de5f3c2cf5f772d827fb1f