diff --git a/src/Ausgabenverwaltung.cpp b/src/Ausgabenverwaltung.cpp
index 2d543b1..ac30ac1 100644
--- a/src/Ausgabenverwaltung.cpp
+++ b/src/Ausgabenverwaltung.cpp
@@ -16,9 +16,18 @@ private:
 	unsigned short month_ = 0;
 	int category_ = 0;
 public:
+	/**
+	 * \brief collection of categories
+	 */
 	static const char* categories[];
+	/**
+	 * \brief collection of months
+	 */
 	static const char* months[];
 
+	/**
+	 * \brief Creates a new instance of invoice
+	 */
 	invoice(const int number, const double amount, const unsigned short month, const int category) : number_(number),
 	                                                                                                 amount_(amount),
 	                                                                                                 month_(month),
@@ -28,36 +37,40 @@ public:
 
 	/**
 	 * \brief Get number of invoice.
-	 * \return number
+	 * \return current number
 	 */
 	int get_number() const
 	{
 		return number_;
 	}
 
+	/**
+	 * \brief get month of invoice
+	 * \return month as index
+	 */
 	unsigned short get_month() const
 	{
 		return month_;
 	}
 
+	/**
+	 * \brief get category of invoice
+	 * \return category as index
+	 */
 	int get_category() const
 	{
 		return category_;
 	}
 
+	/**
+	 * \brief get amount of invoice
+	 * \return amount in euro
+	 */
 	double get_amount() const
 	{
 		return amount_;
 	}
 
-	void print() const
-	{
-		cout << "  Rechnung: " << number_ << endl;
-		cout << "  Betrag: " << amount_ << " Euro" << endl;
-		cout << "  Monat: " << months[month_] << endl;
-		cout << "  Kategorie: " << categories[category_] << endl;
-	}
-
 	/**
  * \brief Implements the equality operator.
  */
@@ -98,6 +111,12 @@ const char* invoice::months[] = {
 	"Dezember"
 };
 
+/**
+ * \brief ask a question and get a answer.
+ * \tparam T return parameter
+ * \param question the question.
+ * \return the value.
+ */
 template <typename T>
 T ask_question(const string& question)
 {
@@ -109,21 +128,35 @@ T ask_question(const string& question)
 }
 
 void create_invoice();
-
+int get_month();
+int get_category();
 void invoices_ordered_by_month();
-
 void delete_invoice();
-
 void sum_invoices_per_category();
 
+/**
+ * \brief compares months ascending.
+ * \param first the first invoice
+ * \param second the second invoice
+ * \return true, if first is smaller than second. False otherwise.
+ */
 bool compare_month_ascending(const invoice& first, const invoice& second)
 {
 	return first.get_month() < second.get_month();
 }
 
+/**
+ * \brief Double linked list of all invoices.
+ */
 list<invoice> invoices;
+/**
+ * \brief Consecutive invoice number
+ */
 int invoice_serial_number;
 
+/**
+ * \brief Start point of application
+ */
 int main()
 {
 	while (true)
@@ -162,9 +195,6 @@ int main()
 	}
 }
 
-int get_month();
-int get_category();
-
 void create_invoice()
 {
 	system("cls");
@@ -178,7 +208,6 @@ void create_invoice()
 	invoices.push_back(i);
 }
 
-
 void invoices_ordered_by_month()
 {
 	system("cls");
@@ -188,7 +217,10 @@ void invoices_ordered_by_month()
 	temp.sort(compare_month_ascending);
 	for (auto invoice : temp)
 	{
-		invoice.print();
+		cout << "  Rechnung: " << invoice.get_number() << endl;
+		cout << "  Betrag: " << invoice.get_amount() << " Euro" << endl;
+		cout << "  Monat: " << invoice::months[invoice.get_month()] << endl;
+		cout << "  Kategorie: " << invoice::categories[invoice.get_category()] << endl;
 		cout << "------------------------------------" << endl;
 	}
 	system("pause");
@@ -196,7 +228,7 @@ void invoices_ordered_by_month()
 
 void delete_invoice()
 {
-	const auto invoice_number = ask_question<int>("Bitte geben Sie die Rechnungsnummer ein");
+	const auto invoice_number = ask_question<int>("Bitte geben Sie eine Rechnungsnummer ein");
 	for (auto invoice : invoices)
 	{
 		if (invoice.get_number() == invoice_number)
@@ -205,7 +237,7 @@ void delete_invoice()
 			return;
 		}
 	}
-	cout << "Rechnung nicht gefunden." << endl;
+	cerr << "Fehler: Rechnung nicht gefunden." << endl;
 	system("pause");
 }
 
@@ -218,8 +250,8 @@ void sum_invoices_per_category()
 	}
 	for (const auto sum : sums)
 	{
-		cout << "    Kategorie: " << invoice::categories[sum.first] << endl;
-		cout << "    Summe: " << sum.second << " Euro" << endl;
+		cout << "  Kategorie: " << invoice::categories[sum.first] << endl;
+		cout << "  Summe: " << sum.second << " Euro" << endl;
 		cout << "------------------------------------" << endl;
 	}
 	system("pause");