diff --git a/src/bfu/widget.h b/src/bfu/widget.h
index 05def3fa..d54695cb 100644
--- a/src/bfu/widget.h
+++ b/src/bfu/widget.h
@@ -51,6 +51,11 @@ struct widget {
 
 struct widget_data {
 	struct widget *widget;
+	/* For WIDGET_FIELD: If CONFIG_UTF8 is defined and UTF-8 I/O
+	 * is enabled for the terminal, then @cdata is in UTF-8;
+	 * otherwise, @cdata is in the charset of the terminal, and
+	 * the charset is assumed to be unibyte.  (Thus, if you choose
+	 * UTF-8 as the charset but disable UTF-8 I/O, you lose.)  */
 	unsigned char *cdata;
 
 	struct box box;
diff --git a/src/document/forms.h b/src/document/forms.h
index 764885d8..55934afe 100644
--- a/src/document/forms.h
+++ b/src/document/forms.h
@@ -94,6 +94,12 @@ struct form_control {
 	int maxlength;
 	int nvalues;
 	unsigned char **values;
+	/* For FC_SELECT: If CONFIG_UTF8 is defined and UTF-8 I/O is
+	 * enabled for the terminal, then @labels is in UTF-8;
+	 * otherwise, @labels is in the charset of the terminal, and
+	 * the charset is assumed to be unibyte.  (Thus, if you choose
+	 * UTF-8 as the charset but disable UTF-8 I/O, you lose.)  The
+	 * charset of the document has no effect here.  */
 	unsigned char **labels;
 	struct menu_item *menu;
 };
diff --git a/src/terminal/draw.h b/src/terminal/draw.h
index aa7d4d66..c5fdfbcc 100644
--- a/src/terminal/draw.h
+++ b/src/terminal/draw.h
@@ -30,7 +30,12 @@ enum screen_char_attr {
 
 /* One position in the terminal screen's image. */
 struct screen_char {
-	/* Contains either character value or frame data. */
+	/* Contains either character value or frame data.
+	 * If CONFIG_UTF8 is defined, and UTF-8 I/O is enabled for the
+	 * terminal, then the character value is in UCS-4; otherwise,
+	 * it is in the charset of the terminal, and the charset is
+	 * assumed to be unibyte.  (Thus, if you choose UTF-8 as the
+	 * charset but disable UTF-8 I/O, you lose.)  */
 #ifdef CONFIG_UTF8
 	unicode_val_T data;
 #else
diff --git a/src/viewer/text/form.h b/src/viewer/text/form.h
index 7691dc0f..bd1d36e1 100644
--- a/src/viewer/text/form.h
+++ b/src/viewer/text/form.h
@@ -43,7 +43,12 @@ struct form_state {
 	 * is not stored anywhere; extending the string always requires
 	 * calling realloc().  The string is not normally allowed to grow
 	 * past @form_control.maxlength bytes (not counting the null),
-	 * but there may be ways to get longer strings.  */
+	 * but there may be ways to get longer strings.  If CONFIG_UTF8
+	 * is defined and UTF-8 I/O is enabled for the terminal, then
+	 * @value is in UTF-8; otherwise, @value is in the charset of the
+	 * terminal, and the charset is assumed to be unibyte.  (Thus, if
+	 * you choose UTF-8 as the charset but disable UTF-8 I/O, you
+	 * lose.)  The charset of the document has no effect here.  */
 	unsigned char *value;
 	/* For FC_TEXT, FC_PASSWORD, and FC_FILE, @state is the byte
 	 * position of the insertion point in @value.