diff --git a/GUI.cpp b/GUI.cpp
index 92358316..8083be78 100644
--- a/GUI.cpp
+++ b/GUI.cpp
@@ -143,6 +143,9 @@ BOOL xg_bShowNumbering = TRUE;
 // キャレットを表示するか？
 BOOL xg_bShowCaret = TRUE;
 
+// 二重マス文字。
+std::wstring xg_strDoubleFrameLetters;
+
 //////////////////////////////////////////////////////////////////////////////
 // static variables
 
@@ -637,6 +640,8 @@ bool __fastcall XgLoadSettings(void)
 
     xg_nViewMode = XG_VIEW_NORMAL;
 
+    xg_strDoubleFrameLetters = XgLoadStringDx1(IDS_DBLFRAME_LETTERS_1);
+
     // 会社名キーを開く。
     MRegKey company_key(HKEY_CURRENT_USER, s_pszSoftwareCompanyName, FALSE);
     if (company_key) {
@@ -840,6 +845,10 @@ bool __fastcall XgLoadSettings(void)
                 }
             }
 
+            if (!app_key.QuerySz(L"DoubleFrameLetters", sz, ARRAYSIZE(sz))) {
+                xg_strDoubleFrameLetters = sz;
+            }
+
             // 保存先のリストを取得する。
             if (!app_key.QueryDword(L"SaveToCount", dwValue)) {
                 nDirCount = dwValue;
@@ -950,6 +959,7 @@ bool __fastcall XgSaveSettings(void)
 
             app_key.SetSz(L"Recent", xg_dict_name.c_str());
             app_key.SetSz(L"BlackCellImage", xg_strBlackCellImage.c_str());
+            app_key.SetSz(L"DoubleFrameLetters", xg_strDoubleFrameLetters.c_str());
 
             // 保存先のリストを設定する。
             nCount = static_cast<int>(s_dirs_save_to.size());
@@ -5365,6 +5375,13 @@ BOOL SettingsDlg_OnInitDialog(HWND hwnd)
         ComboBox_SetCurSel(hCmb1, ComboBox_FindStringExact(hCmb1, -1, psz));
     }
 
+    // 二重マス文字。
+    HWND hCmb2 = GetDlgItem(hwnd, cmb2);
+    for (INT i = IDS_DBLFRAME_LETTERS_1; i <= IDS_DBLFRAME_LETTERS_6; ++i) {
+        ComboBox_AddString(hCmb2, XgLoadStringDx1(i));
+    }
+    ComboBox_SetText(hCmb2, xg_strDoubleFrameLetters.c_str());
+
     UpdateBlockPreview(hwnd);
 
     return TRUE;
@@ -5501,6 +5518,11 @@ void SettingsDlg_OnOK(HWND hwnd)
         ::PostMessageW(xg_hHintsWnd, WM_SIZE, 0, 0);
     }
 
+    // 二重マス文字。
+    WCHAR szText[MAX_PATH];
+    ComboBox_GetText(GetDlgItem(hwnd, cmb2), szText, _countof(szText));
+    xg_strDoubleFrameLetters = szText;
+
     // イメージを更新する。
     int x = XgGetHScrollPos();
     int y = XgGetVScrollPos();
diff --git a/HISTORY.txt b/HISTORY.txt
index b39af568..c2d722d9 100644
--- a/HISTORY.txt
+++ b/HISTORY.txt
@@ -418,6 +418,7 @@
 	蟇ｾ遘ｰ縺ｮ繝ｫ繝ｼ繝ｫ縺ｮ縺ｨ縺阪↓縲瑚ｧ｣繧呈ｱゅａ繧�(鮟偵�槭せ霑ｽ蜉�縺ｪ縺�)縲阪〒逶､繧貞ｯｾ遘ｰ縺ｫ縺吶ｋ繧医≧縺ｫ縺励◆縲�
 	譛�螟ｧ蜊倩ｪ樣聞蜻ｨ繧翫ｒ菫ｮ豁｣縲�
 	縲茎keleton縲阪�ｮtypo繧剃ｿｮ豁｣縲�
+	縲御ｺ碁�阪�槭せ譁�蟄励�阪�ｮ謖�螳壹′蜿ｯ閭ｽ縺ｫ縲�
 
 ## History of XWordGiver (English)
 
@@ -471,3 +472,4 @@
     - When the policy is symmetrical, "Finding solution (without adding blocks)" forces the board symmetrical.
     - Fixed around Max Word Length.
     - Fixed typo of skeleton.
+    - Enabled the "double-frame letters" settings.
diff --git a/lang/en_US.rc b/lang/en_US.rc
index b3363c18..a7eebbfb 100644
--- a/lang/en_US.rc
+++ b/lang/en_US.rc
@@ -562,7 +562,7 @@ FONT 9, "Tahoma"
     PUSHBUTTON "Cancel", IDCANCEL, 240, 135, 60, 14
 }
 
-IDD_CONFIG DIALOG 0, 0, 274, 211
+IDD_CONFIG DIALOG 0, 0, 274, 250
 CAPTION "Configuration"
 STYLE DS_CENTER | DS_MODALFRAME | WS_VISIBLE | WS_CAPTION | WS_SYSMENU
 FONT 9, "Tahoma"
@@ -600,11 +600,13 @@ FONT 9, "Tahoma"
     PUSHBUTTON "Reset", psh11, 209, 140, 35, 14
     LTEXT "Block i&mage:", -1, 5, 166, 52, 11
     COMBOBOX cmb1, 60, 165, 102, 300, CBS_HASSTRINGS | CBS_AUTOHSCROLL | CBS_DROPDOWNLIST | WS_VSCROLL | WS_TABSTOP
-    DEFPUSHBUTTON "OK", IDOK, 145, 192, 60, 14
-    PUSHBUTTON "Cancel", IDCANCEL, 210, 192, 60, 14
     LTEXT "Please drop your favorite square image into the BLOCK folder in BMP/EMF format.", -1, 5, 182, 138, 21
     CONTROL "", ico1, "STATIC", SS_BITMAP, 165, 165, 20, 20
     CONTROL "", ico2, "STATIC", SS_ENHMETAFILE, 165, 165, 20, 20
+    RTEXT "&Double-frame letters:", -1, 5, 212, 80, 12
+    COMBOBOX cmb2, 90, 210, 180, 300, CBS_HASSTRINGS | CBS_AUTOHSCROLL | CBS_DROPDOWN | WS_VSCROLL | WS_TABSTOP
+    DEFPUSHBUTTON "OK", IDOK, 145, 230, 60, 14
+    PUSHBUTTON "Cancel", IDCANCEL, 210, 230, 60, 14
 }
 
 IDD_SEQSOLVE DIALOG 0, 0, 310, 80
@@ -1468,6 +1470,12 @@ STRINGTABLE
     IDS_SHALLIMIRROR, "Shall I make the block pattern symmetrical according to the specified policy?"
     IDS_EVENROWLINESYMV, "Forcing symmetrical even rows and banning adjacent blocks is unreasonable. Please reconfirm the policies."
     IDS_EVENCOLLINESYMH, "Forcing symmetrical even columns and banning adjacent blocks is unreasonable. Please reconfirm the policies."
+    IDS_DBLFRAME_LETTERS_1, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+    IDS_DBLFRAME_LETTERS_2, "ﾎｱﾎｲﾎｳﾎｴﾎｵﾎｶﾎｷﾎｸﾎｹﾎｺﾎｻﾎｼﾎｽﾎｾﾎｿﾏ�ﾏ�ﾏρ�ﾏ�ﾏ�ﾏ�ﾏ委火対槻鳶釆偏茂緑佩厥墅嶄慚斟榻湮�ﾎ｡ﾎ｣ﾎ､ﾎ･ﾎｦﾎｧﾎｨﾎｩ"
+    IDS_DBLFRAME_LETTERS_3, "縺ゅ＞縺�縺医♀縺九″縺上¢縺薙＆縺励☆縺帙◎縺溘■縺､縺ｦ縺ｨ縺ｪ縺ｫ縺ｬ縺ｭ縺ｮ縺ｯ縺ｲ縺ｵ縺ｸ縺ｻ縺ｾ縺ｿ繧�繧√ｂ繧�繧�繧医ｉ繧翫ｋ繧後ｍ繧上ｒ繧�"
+    IDS_DBLFRAME_LETTERS_4, "繧｢繧､繧ｦ繧ｨ繧ｪ繧ｫ繧ｭ繧ｯ繧ｱ繧ｳ繧ｵ繧ｷ繧ｹ繧ｻ繧ｽ繧ｿ繝√ヤ繝�繝医リ繝九レ繝阪ヮ繝上ヲ繝輔�倥�帙�槭Α繝�繝｡繝｢繝､繝ｦ繝ｨ繝ｩ繝ｪ繝ｫ繝ｬ繝ｭ繝ｯ繝ｲ繝ｳ"
+    IDS_DBLFRAME_LETTERS_5, "縺�繧阪�ｯ縺ｫ縺ｻ縺ｸ縺ｨ縺｡繧翫〓繧九ｒ繧上°繧医◆繧後◎縺､縺ｭ縺ｪ繧峨��縺�繧舌�ｮ縺翫￥繧�縺ｾ縺代�ｵ縺薙∴縺ｦ縺ゅ＆縺阪ｆ繧√∩縺励ｑ縺ｲ繧ゅ○縺吶ｓ"
+    IDS_DBLFRAME_LETTERS_6, "繧､繝ｭ繝上ル繝帙�倥ヨ繝√Μ繝後Ν繝ｲ繝ｯ繧ｫ繝ｨ繧ｿ繝ｬ繧ｽ繝�繝阪リ繝ｩ繝�繧ｦ繝ｰ繝弱が繧ｯ繝､繝槭こ繝輔さ繧ｨ繝�繧｢繧ｵ繧ｭ繝ｦ繝｡繝溘す繝ｱ繝偵Δ繧ｻ繧ｹ繝ｳ"
     IDS_TT_NEW, "New crossword"
     IDS_TT_GENERATE, "Generate crossword"
     IDS_TT_OPEN, "Open crossword"
diff --git a/lang/ja_JP.rc b/lang/ja_JP.rc
index 47bfde7b..d62a4b71 100644
--- a/lang/ja_JP.rc
+++ b/lang/ja_JP.rc
@@ -565,7 +565,7 @@ FONT 9, "MS UI Gothic"
     PUSHBUTTON "繧ｭ繝｣繝ｳ繧ｻ繝ｫ", IDCANCEL, 240, 135, 60, 14
 }
 
-IDD_CONFIG DIALOG 0, 0, 274, 211
+IDD_CONFIG DIALOG 0, 0, 274, 250
 CAPTION "繧ｯ繝ｭ繧ｹ繝ｯ繝ｼ繝� 繧ｮ繝舌�ｼ縺ｮ險ｭ螳�"
 STYLE DS_CENTER | DS_MODALFRAME | WS_VISIBLE | WS_CAPTION | WS_SYSMENU
 FONT 9, "MS UI Gothic"
@@ -603,11 +603,13 @@ FONT 9, "MS UI Gothic"
     PUSHBUTTON "繝ｪ繧ｻ繝�繝�", psh11, 209, 140, 35, 14
     LTEXT "鮟偵�槭せ逕ｻ蜒�(&M):", -1, 5, 166, 52, 11
     COMBOBOX cmb1, 60, 165, 102, 300, CBS_HASSTRINGS | CBS_AUTOHSCROLL | CBS_DROPDOWNLIST | WS_VSCROLL | WS_TABSTOP
-    DEFPUSHBUTTON "OK", IDOK, 145, 192, 60, 14
-    PUSHBUTTON "繧ｭ繝｣繝ｳ繧ｻ繝ｫ", IDCANCEL, 210, 192, 60, 14
     LTEXT "BLOCK繝輔か繝ｫ繝�縺ｫ螂ｽ縺阪↑豁｣譁ｹ蠖｢縺ｮ逕ｻ蜒上ｒBMP/EMF蠖｢蠑上〒謾ｾ繧願ｾｼ繧薙〒荳九＆縺�縲�", -1, 10, 182, 128, 21
     CONTROL "", ico1, "STATIC", SS_BITMAP, 165, 165, 20, 20
     CONTROL "", ico2, "STATIC", SS_ENHMETAFILE, 165, 165, 20, 20
+    RTEXT "莠碁�阪�槭せ譁�蟄�(&D):", -1, 5, 212, 80, 12
+    COMBOBOX cmb2, 90, 210, 180, 300, CBS_HASSTRINGS | CBS_AUTOHSCROLL | CBS_DROPDOWN | WS_VSCROLL | WS_TABSTOP
+    DEFPUSHBUTTON "OK", IDOK, 145, 230, 60, 14
+    PUSHBUTTON "繧ｭ繝｣繝ｳ繧ｻ繝ｫ", IDCANCEL, 210, 230, 60, 14
 }
 
 IDD_SEQSOLVE DIALOG 0, 0, 310, 80
@@ -1471,6 +1473,12 @@ STRINGTABLE
     IDS_SHALLIMIRROR, "謖�螳壹＆繧後◆繝ｫ繝ｼ繝ｫ縺ｫ蠕薙▲縺ｦ鮟偵�槭せ繝代ち繝ｼ繝ｳ繧貞ｯｾ遘ｰ縺ｫ縺励∪縺吶°�ｼ�"
     IDS_EVENROWLINESYMV, "蛛ｶ謨ｰ陦梧焚縺ｧ繧ｿ繝�邱壼ｯｾ遘ｰ縺ｧ騾｣鮟堤ｦ√�ｯ荳榊庄縺ｧ縺吶�ゅΝ繝ｼ繝ｫ繧貞�咲｢ｺ隱阪＠縺ｦ荳九＆縺�縲�"
     IDS_EVENCOLLINESYMH, "蛛ｶ謨ｰ蛻玲焚縺ｧ繝ｨ繧ｳ邱壼ｯｾ遘ｰ縺ｧ騾｣鮟堤ｦ√�ｯ荳榊庄縺ｧ縺吶�ゅΝ繝ｼ繝ｫ繧貞�咲｢ｺ隱阪＠縺ｦ荳九＆縺�縲�"
+    IDS_DBLFRAME_LETTERS_1, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+    IDS_DBLFRAME_LETTERS_2, "ﾎｱﾎｲﾎｳﾎｴﾎｵﾎｶﾎｷﾎｸﾎｹﾎｺﾎｻﾎｼﾎｽﾎｾﾎｿﾏ�ﾏ�ﾏρ�ﾏ�ﾏ�ﾏ�ﾏ委火対槻鳶釆偏茂緑佩厥墅嶄慚斟榻湮�ﾎ｡ﾎ｣ﾎ､ﾎ･ﾎｦﾎｧﾎｨﾎｩ"
+    IDS_DBLFRAME_LETTERS_3, "縺ゅ＞縺�縺医♀縺九″縺上¢縺薙＆縺励☆縺帙◎縺溘■縺､縺ｦ縺ｨ縺ｪ縺ｫ縺ｬ縺ｭ縺ｮ縺ｯ縺ｲ縺ｵ縺ｸ縺ｻ縺ｾ縺ｿ繧�繧√ｂ繧�繧�繧医ｉ繧翫ｋ繧後ｍ繧上ｒ繧�"
+    IDS_DBLFRAME_LETTERS_4, "繧｢繧､繧ｦ繧ｨ繧ｪ繧ｫ繧ｭ繧ｯ繧ｱ繧ｳ繧ｵ繧ｷ繧ｹ繧ｻ繧ｽ繧ｿ繝√ヤ繝�繝医リ繝九レ繝阪ヮ繝上ヲ繝輔�倥�帙�槭Α繝�繝｡繝｢繝､繝ｦ繝ｨ繝ｩ繝ｪ繝ｫ繝ｬ繝ｭ繝ｯ繝ｲ繝ｳ"
+    IDS_DBLFRAME_LETTERS_5, "縺�繧阪�ｯ縺ｫ縺ｻ縺ｸ縺ｨ縺｡繧翫〓繧九ｒ繧上°繧医◆繧後◎縺､縺ｭ縺ｪ繧峨��縺�繧舌�ｮ縺翫￥繧�縺ｾ縺代�ｵ縺薙∴縺ｦ縺ゅ＆縺阪ｆ繧√∩縺励ｑ縺ｲ繧ゅ○縺吶ｓ"
+    IDS_DBLFRAME_LETTERS_6, "繧､繝ｭ繝上ル繝帙�倥ヨ繝√Μ繝後Ν繝ｲ繝ｯ繧ｫ繝ｨ繧ｿ繝ｬ繧ｽ繝�繝阪リ繝ｩ繝�繧ｦ繝ｰ繝弱が繧ｯ繝､繝槭こ繝輔さ繧ｨ繝�繧｢繧ｵ繧ｭ繝ｦ繝｡繝溘す繝ｱ繝偵Δ繧ｻ繧ｹ繝ｳ"
     IDS_TT_NEW, "譁ｰ隕丈ｽ懈��"
     IDS_TT_GENERATE, "蝠城｡後ｒ閾ｪ蜍慕函謌舌☆繧�"
     IDS_TT_OPEN, "蝠城｡後ｒ髢九￥"
diff --git a/resource.h b/resource.h
index b928425f..e81424dd 100644
--- a/resource.h
+++ b/resource.h
@@ -168,6 +168,12 @@
 #define IDS_SHALLIMIRROR                    260
 #define IDS_EVENROWLINESYMV                 261
 #define IDS_EVENCOLLINESYMH                 262
+#define IDS_DBLFRAME_LETTERS_1              263
+#define IDS_DBLFRAME_LETTERS_2              264
+#define IDS_DBLFRAME_LETTERS_3              265
+#define IDS_DBLFRAME_LETTERS_4              266
+#define IDS_DBLFRAME_LETTERS_5              267
+#define IDS_DBLFRAME_LETTERS_6              268
 #define IDS_TT_NEW                          10100
 #define IDS_TT_GENERATE                     10101
 #define IDS_TT_OPEN                         10102
diff --git a/xword.cpp b/xword.cpp
index a661459e..8049bc1b 100644
--- a/xword.cpp
+++ b/xword.cpp
@@ -3631,7 +3631,10 @@ void __fastcall XgDrawMarkWord(HDC hdc, LPSIZE psiz)
         }
         ::InflateRect(&rc, 4, 4);
 
-        StringCbPrintf(sz, sizeof(sz), L"%c", ZEN_LARGE_A + i);
+        if (i < (INT)xg_strDoubleFrameLetters.size())
+            StringCbPrintf(sz, sizeof(sz), L"%c", xg_strDoubleFrameLetters[i]);
+        else
+            StringCbPrintf(sz, sizeof(sz), L"%c", ZEN_BLACK);
         ::GetTextExtentPoint32W(hdc, sz, int(wcslen(sz)), &siz);
 
         RECT rcText = rc;
@@ -3892,7 +3895,10 @@ void __fastcall XgDrawXWord_NormalView(XG_Board& xw, HDC hdc, LPSIZE psiz, bool
                 ::InflateRect(&rc, 4, 4);
             }
 
-            StringCbPrintf(sz, sizeof(sz), L"%c", L'A' + nMarked);
+            if (nMarked < (INT)xg_strDoubleFrameLetters.size())
+                StringCbPrintf(sz, sizeof(sz), L"%c", xg_strDoubleFrameLetters[nMarked]);
+            else
+                StringCbPrintf(sz, sizeof(sz), L"%c", ZEN_BLACK);
 
             // 二重マスの右下端の文字の背景を塗りつぶす。
             RECT rcText;
diff --git a/xword.hpp b/xword.hpp
index 181f51fb..a71f2cbf 100644
--- a/xword.hpp
+++ b/xword.hpp
@@ -795,6 +795,8 @@ inline BOOL XgIsUserJapanese(VOID)
 extern BOOL xg_bShowNumbering;
 // キャレットを表示するか？
 extern BOOL xg_bShowCaret;
+// 二重マス文字。
+extern std::wstring xg_strDoubleFrameLetters;
 
 //////////////////////////////////////////////////////////////////////////////