diff --git a/native/src/additional_proximity_chars.cpp b/native/src/additional_proximity_chars.cpp
new file mode 100644
index 0000000..74c96ae
--- /dev/null
+++ b/native/src/additional_proximity_chars.cpp
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2012 The Android Open Source Project
+ *
+ * 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
+ *
+ *      http://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.
+ */
+
+#include "additional_proximity_chars.h"
+
+namespace latinime {
+const std::string AdditionalProximityChars::LOCALE_EN_US("en");
+
+const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_A[EN_US_ADDITIONAL_A_SIZE] = {
+    'e', 'i', 'o', 'u'
+};
+
+const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_E[EN_US_ADDITIONAL_E_SIZE] = {
+    'a', 'i', 'o', 'u'
+};
+
+const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_I[EN_US_ADDITIONAL_I_SIZE] = {
+    'a', 'e', 'o', 'u'
+};
+
+const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_O[EN_US_ADDITIONAL_O_SIZE] = {
+    'a', 'e', 'i', 'u'
+};
+
+const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_U[EN_US_ADDITIONAL_U_SIZE] = {
+    'a', 'e', 'i', 'o'
+};
+}
diff --git a/native/src/additional_proximity_chars.h b/native/src/additional_proximity_chars.h
new file mode 100644
index 0000000..b76726c
--- /dev/null
+++ b/native/src/additional_proximity_chars.h
@@ -0,0 +1,92 @@
+/*
+ * Copyright (C) 2012 The Android Open Source Project
+ *
+ * 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
+ *
+ *      http://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.
+ */
+
+#ifndef LATINIME_ADDITIONAL_PROXIMITY_CHARS_H
+#define LATINIME_ADDITIONAL_PROXIMITY_CHARS_H
+
+#include <stdint.h>
+#include <string>
+
+namespace latinime {
+
+class AdditionalProximityChars {
+ private:
+    static const std::string LOCALE_EN_US;
+    static const int EN_US_ADDITIONAL_A_SIZE = 4;
+    static const uint16_t EN_US_ADDITIONAL_A[];
+    static const int EN_US_ADDITIONAL_E_SIZE = 4;
+    static const uint16_t EN_US_ADDITIONAL_E[];
+    static const int EN_US_ADDITIONAL_I_SIZE = 4;
+    static const uint16_t EN_US_ADDITIONAL_I[];
+    static const int EN_US_ADDITIONAL_O_SIZE = 4;
+    static const uint16_t EN_US_ADDITIONAL_O[];
+    static const int EN_US_ADDITIONAL_U_SIZE = 4;
+    static const uint16_t EN_US_ADDITIONAL_U[];
+
+    static bool isEnLocale(std::string* locale_str) {
+        return locale_str && locale_str->size() >= LOCALE_EN_US.size()
+                && locale_str->compare(0, LOCALE_EN_US.size(), LOCALE_EN_US);
+    }
+
+ public:
+    static int getAdditionalCharsSize(std::string* locale_str, uint16_t c) {
+        if (!isEnLocale(locale_str)) {
+            return 0;
+        }
+        switch(c) {
+        case 'a':
+            return EN_US_ADDITIONAL_A_SIZE;
+        case 'e':
+            return EN_US_ADDITIONAL_E_SIZE;
+        case 'i':
+            return EN_US_ADDITIONAL_I_SIZE;
+        case 'o':
+            return EN_US_ADDITIONAL_O_SIZE;
+        case 'u':
+            return EN_US_ADDITIONAL_U_SIZE;
+        default:
+            return 0;
+        }
+    }
+
+    static const uint16_t* getAdditionalChars(std::string* locale_str, uint16_t c) {
+        if (!isEnLocale(locale_str)) {
+            return 0;
+        }
+        switch(c) {
+        case 'a':
+            return EN_US_ADDITIONAL_A;
+        case 'e':
+            return EN_US_ADDITIONAL_E;
+        case 'i':
+            return EN_US_ADDITIONAL_I;
+        case 'o':
+            return EN_US_ADDITIONAL_O;
+        case 'u':
+            return EN_US_ADDITIONAL_U;
+        default:
+            return 0;
+        }
+    }
+
+    static bool hasAdditionalChars(std::string* locale_str, uint16_t c) {
+        return getAdditionalCharsSize(locale_str, c) > 0;
+    }
+};
+
+}
+
+#endif // LATINIME_ADDITIONAL_PROXIMITY_CHARS_H
diff --git a/native/src/proximity_info.cpp b/native/src/proximity_info.cpp
index ad19f58..561fffa 100644
--- a/native/src/proximity_info.cpp
+++ b/native/src/proximity_info.cpp
@@ -16,10 +16,11 @@
 
 #include <assert.h>
 #include <stdio.h>
-#include <string.h>
+#include <string>
 
 #define LOG_TAG "LatinIME: proximity_info.cpp"
 
+#include "additional_proximity_chars.h"
 #include "dictionary.h"
 #include "proximity_info.h"
 
@@ -33,9 +34,9 @@
     }
 }
 
-ProximityInfo::ProximityInfo(const int maxProximityCharsSize, const int keyboardWidth,
-        const int keyboardHeight, const int gridWidth, const int gridHeight,
-        const int mostCommonKeyWidth,
+ProximityInfo::ProximityInfo(const std::string localeStr, const int maxProximityCharsSize,
+        const int keyboardWidth, const int keyboardHeight, const int gridWidth,
+        const int gridHeight, const int mostCommonKeyWidth,
         const uint32_t *proximityCharsArray, const int keyCount, const int32_t *keyXCoordinates,
         const int32_t *keyYCoordinates, const int32_t *keyWidths, const int32_t *keyHeights,
         const int32_t *keyCharCodes, const float *sweetSpotCenterXs, const float *sweetSpotCenterYs,
diff --git a/native/src/proximity_info.h b/native/src/proximity_info.h
index caabadf..bfcef63 100644
--- a/native/src/proximity_info.h
+++ b/native/src/proximity_info.h
@@ -18,6 +18,7 @@
 #define LATINIME_PROXIMITY_INFO_H
 
 #include <stdint.h>
+#include <string>
 
 #include "defines.h"
 
@@ -43,9 +44,9 @@
         ADDITIONAL_PROXIMITY_CHAR
     } ProximityType;
 
-    ProximityInfo(const int maxProximityCharsSize, const int keyboardWidth,
-            const int keybaordHeight, const int gridWidth, const int gridHeight,
-            const int mostCommonkeyWidth,
+    ProximityInfo(const std::string localeStr, const int maxProximityCharsSize,
+            const int keyboardWidth, const int keybaordHeight, const int gridWidth,
+            const int gridHeight, const int mostCommonkeyWidth,
             const uint32_t *proximityCharsArray, const int keyCount, const int32_t *keyXCoordinates,
             const int32_t *keyYCoordinates, const int32_t *keyWidths, const int32_t *keyHeights,
             const int32_t *keyCharCodes, const float *sweetSpotCenterXs,