A brief history of the "number", also called "initial", is in order:
"Putting numbers on linen is a question of coquetry. These common or worked initials, whose fantasy comes to add to the richness of the linen, it is a seal of elegance and research: it is the definitive seal which indicates the perfection of a work.
There are two expressions used to designate the various forms of these embroideries. The initial is called encryptedwhen it is embroidered; it is called markedwhen it is made in cross stitch. The beautiful linen of body or house is figured, whereas the current linen is marked.
Which initials to choose? - Linen is marked with the initials of your first name and surname.
The place occupied by the initials in figures. Body linen. - Women's shirts are figured on the left, on the chest, a little before the underarm. In some fancy trousseaux, we see the initials reversed (towards oneself) embroidered on the left wrist
White cotton is mainly used for the ciphering of "elegant" linen; sometimes coloured cottons are also used, harmonised with the colours of the designs. The variety of monograms can thus be multiplied by differentiating the colours; for example, two letters will be of different colours, with a background of a third colour. An elegant set is given by a dark blue letter, another orange, on an azure blue background.
Marker stitch letters are usually made with red cotton."
Chiffrer et Marquer, Album d'alphabets et de monogrammes - N°2 (c. 1920) Edition of the Petit Écho de la Mode.
