diff --git a/QElephant/QuBit.py b/QElephant/QuBit.py
index 455510d..58cac2a 100644
--- a/QElephant/QuBit.py
+++ b/QElephant/QuBit.py
@@ -71,11 +71,11 @@ class MuBit:
             raise TypeError(f"MuBit indices must be integers, not {type(item)}")
         if item > self.__n:
             raise IndexError("MuBit index out of range")
-
         item = item%self.__n
+
         return IQuBit(item, self)
 
-    def __apply(self, i: int, matrix: Matrix) -> None:
+    def __apply(self, matrix: Matrix, i: int) -> None:
         if type(i) is not int:
             raise TypeError(f"MuBit indices must be integers, not {type(i)}")
         if i > self.__n:
diff --git a/docs/QuBit/MuBit.md b/docs/QuBit/MuBit.md
index b0c586f..774f4fa 100644
--- a/docs/QuBit/MuBit.md
+++ b/docs/QuBit/MuBit.md
@@ -40,13 +40,13 @@ MuBit(n: int)
 
   creates an intricated QuBit corresponding to the item-th QuBit.
 
-- ```__apply(m: Matrix) -> None```
+- ```__apply(matrix: Matrix, i: int) -> None```
 
-  takes a Matrix and modifies the QuBits according the matrix.
+  takes a Matrix of size (2, 2) and modifies the i-th QuBits according the matrix.
 
-- ```__mapply(m: Matrix, i: int) -> None```
+- ```__mapply(matrix: Matrix, i: int) -> None```
 
-  takes a Matrix and modifies the i-th QuBit according the matrix.
+  takes a Matrix aof size (4, 4) and modifies the i-th QuBit and le next one together according the matrix.
 
 - ```__getProbs(i: int) -> float```