diff --git a/data/insee_data.go b/data/insee_data.go
index a9501d7..5bf2b66 100644
--- a/data/insee_data.go
+++ b/data/insee_data.go
@@ -40,7 +40,7 @@ type InseeData struct {
 
 func NewInseeData(inseeNumber string) (*InseeData, error) {
 	if len(inseeNumber) != 15 {
-		return nil, fmt.Errorf("provided insee number must contain 15 characters")
+		return nil, fmt.Errorf("le numéro INSEE number must contain 15 characters")
 	}
 	num := inseeNumber
 	departmentCode := num[5:7]
diff --git a/main.go b/main.go
index 142768b..20adb9b 100644
--- a/main.go
+++ b/main.go
@@ -4,19 +4,27 @@ import (
 	"flag"
 	"fmt"
 	"insee_number_translator/data"
+	"os"
 )
 
 func main() {
-	var preProcess bool
-	flag.BoolVar(&preProcess, "pre-process", false, "Pre-process data files.")
+	flag.Usage = func() {
+		out := flag.CommandLine.Output()
+		fmt.Fprintf(out, "Usage: %s [flags] [numero_insee...]\n", os.Args[0])
+		fmt.Fprintf(out, "\nCe programme décode les informations contenues dans votre numéro INSEE (numéro de sécurité sociale français)")
+		fmt.Fprintf(out, "et vous les affiche d'une manière lisible et claire.\n")
+		flag.PrintDefaults()
+		fmt.Fprintf(out, "\nLes arguments numero_insee doivent comporter 15 caractères. Il est possible d'en spécifier plusieurs séparés par un espace.\n")
+	}
+
 	flag.Parse()
 
-	if preProcess {
-		data.PreProcessRawData("data/raw_data", "data/curated_data")
+	numbers := flag.Args()
+	if len(numbers) == 0 {
+		flag.Usage()
 		return
 	}
 
-	numbers := flag.Args()
 	for _, number := range numbers {
 		insee, err := data.NewInseeData(number)
 		if err != nil {
diff --git a/data/preprocess.go b/pre_process/preprocess.go
similarity index 79%
rename from data/preprocess.go
rename to pre_process/preprocess.go
index 7c04e63..ffb7a68 100644
--- a/data/preprocess.go
+++ b/pre_process/preprocess.go
@@ -1,4 +1,4 @@
-package data
+package main
 
 import (
 	"encoding/csv"
@@ -8,27 +8,33 @@ import (
 	"os"
 )
 
+func main() {
+	fmt.Println("This is intended for tinkerers only, not for end users.")
+	PreProcessRawData("data/raw_data", "data/curated_data")
+}
+
 func PreProcessRawData(sourceFolder, targetFolder string) {
 	err := os.MkdirAll(targetFolder, 0755)
 	if err != nil {
 		fmt.Printf("Error, couldn't create target folder %s: %s", targetFolder, err)
 		return
 	}
-	err = preProcessCities(sourceFolder+"/commune2021.csv", targetFolder+"/cities.json")
+	err = preProcessCities(sourceFolder+"/commune.csv", targetFolder+"/cities.json")
 	if err != nil {
-		fmt.Println("Error during cities pre processing")
-		fmt.Println(err.Error())
+		fmt.Fprintln(os.Stderr, "Error during cities pre processing")
+		fmt.Fprintln(os.Stderr, err.Error())
 	}
-	err = preProcessDepartments(sourceFolder+"/departement2021.csv", targetFolder+"/departments.json")
+	err = preProcessDepartments(sourceFolder+"/departement.csv", targetFolder+"/departments.json")
 	if err != nil {
-		fmt.Println("Error during departments pre processing")
-		fmt.Println(err.Error())
+		fmt.Fprintln(os.Stderr, "Error during departments pre processing")
+		fmt.Fprintln(os.Stderr, err.Error())
 	}
-	err = preProcessCountries(sourceFolder+"/pays2021.csv", targetFolder+"/countries.json")
+	err = preProcessCountries(sourceFolder+"/pays.csv", targetFolder+"/countries.json")
 	if err != nil {
-		fmt.Println("Error during countries pre processing")
-		fmt.Println(err.Error())
+		fmt.Fprintln(os.Stderr, "Error during countries pre processing")
+		fmt.Fprintln(os.Stderr, err.Error())
 	}
+	fmt.Println("Finished pre-processing data.")
 }
 
 func preProcessCities(sourceFileName, targetFileName string) error {