diff --git a/src/lib/sedlexing.mli b/src/lib/sedlexing.mli
index 55c439d..5d6d801 100644
--- a/src/lib/sedlexing.mli
+++ b/src/lib/sedlexing.mli
@@ -45,15 +45,18 @@ exception MalFormed
         Uchars [a], a position [pos] and a code point count [n].  The
         function should put [n] code points or less in [a], starting at
         position [pos], and return the number of characters provided. A
-        return value of 0 means end of input. *)
+        return value of 0 means end of input. [bytes_per_char] argument is
+        optional. If unspecified, byte positions are the same as code point
+        position. *)
 val create :
   ?bytes_per_char:(Uchar.t -> int) ->
   (Uchar.t array -> int -> int -> int) ->
   lexbuf
 
-(** set the initial tracked input position for [lexbuf].
+(** set the initial tracked input position, in code point, for [lexbuf].
         If set to [Lexing.dummy_pos], Sedlexing will not track position
-        information for you. *)
+        information for you. [bytes_position] is optional. If unspecified,
+        byte postion is set to the same value as code point position. *)
 val set_position :
   ?bytes_position:Lexing.position -> lexbuf -> Lexing.position -> unit
 
@@ -62,13 +65,16 @@ val set_position :
         returned {!Lexing.position} records. *)
 val set_filename : lexbuf -> string -> unit
 
-(** Create a lexbuf from a stream of Unicode code points. *)
+(** Create a lexbuf from a stream of Unicode code points. [bytes_per_char] is
+    optional. If unspecified, byte positions are the same as code point positions. *)
 val from_gen : ?bytes_per_char:(Uchar.t -> int) -> Uchar.t Gen.t -> lexbuf
 
-(** Create a lexbuf from an array of Unicode code points. *)
+(** Create a lexbuf from an array of Unicode code points. [bytes_per_char] is
+    optional. If unspecified, byte positions are the same as code point positions. *)
 val from_int_array : ?bytes_per_char:(Uchar.t -> int) -> int array -> lexbuf
 
-(** Create a lexbuf from an array of Unicode code points. *)
+(** Create a lexbuf from an array of Unicode code points. [bytes_per_char] is
+    optional. If unspecified, byte positions are the same as code point positions. *)
 val from_uchar_array :
   ?bytes_per_char:(Uchar.t -> int) -> Uchar.t array -> lexbuf