diff --git a/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md b/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md
index 36bd76eeb..5c083bcdc 100644
--- a/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md
+++ b/00.brainstorming-pedagogical-teams/40.collection-existing-pedagogical-content/04.reference-frames-coordinate-systems/textbook.fr.md
@@ -226,6 +226,12 @@ In Cartesian coordinates, the base vectors keep the
 <br>$`(\overrightarrow{e_x},\overrightarrow{e_y},\overrightarrow{e_z})`$ 
 base ortogonal independiente de la posición de $`M`$ / base orthogonale indépendante
 de la position de $`M`$ / orthogonal basis independent of the position of $`M`$.
+<br>$`||\overrightarrow{e_x}||=||\overrightarrow{e_y}||=||\overrightarrow{e_z}||=1`$<br>
+$`\overrightarrow{e_x}\perp\overrightarrow{e_y}\quad,\quad\overrightarrow{e_y}
+\perp\overrightarrow{e_z}\quad,\quad\overrightarrow{e_x}\perp\overrightarrow{e_z}`$<br>
+<br>$`(\overrightarrow{e_x},\overrightarrow{e_y},\overrightarrow{e_z})`$ 
+base ortogonal independiente de la posición de $`M`$ / base orthogonale indépendante
+de la position de $`M`$ / orthogonal basis independent of the position of $`M`$.
 
 * **N3 ($`\rightarrow`$ N4)**<br>
 [ES] La norma del vector $`\partial\overrightarrow{OM}_x=\overrightarrow{dl_x}`$