Method for determining the location of marks (24, 25, 26, 27) to be made on the external face of the skin of a patient intended to receive a prosthesis, for example for treating a hernia defect, comprising the following steps: i) a device for selecting the size and shape of a prosthesis and/or for determining the location of marks (24, 25, 26, 27) to be made on the external face of the skin of a patient intended to receive such a prosthesis is provided, said device comprising at least one sheet of transparent material, the said sheet being of elongate overall shape defining a longitudinal direction, a transverse direction, and a centre of the said sheet, the said sheet comprising: - at least one plurality of graphical representations (3a, 3b, 4a, 4b, 5a, 5b, 5c, 5d) concentrically embodying the respective outlines of various shapes and sizes of prostheses, - at least one plurality of first holes (6a, 6b, 6c, 6d, 6e, 7a, 7b, 7c, 7d, 7e) arranged along the transverse line passing through the centre of the said sheet, and - at least one plurality of second holes (8a, 8b, 8c, 8d, 8e, 8f, 8g, 9a, 9b, 9c, 9d, 9e, 9f, 9g) arranged along the longitudinal line passing through the centre of the said sheet, - ii) the device is applied to the external face of the region of the skin of the patient, for example under which the hernia defect is located, and the centre of the sheet of material of the device is substantially aligned on the site of the hernia defect, - iii) the sheet of material is pivoted so as to select the graphical representation corresponding to a size and shape of a prosthesis constituting the best coverage of the hernia defect, iv) using the first and second holes of the device, the said skin is marked in the transverse direction of the sheet and in the longitudinal direction of the sheet. i) a device for selecting the size and shape of a prosthesis and/or for determining the location of marks (24, 25, 26, 27) to be made on the external face of the skin of a patient intended to receive such a prosthesis is provided, said device comprising at least one sheet of transparent material, the said sheet being of elongate overall shape defining a longitudinal direction, a transverse direction, and a centre of the said sheet, the said sheet comprising: - at least one plurality of graphical representations (3a, 3b, 4a, 4b, 5a, 5b, 5c, 5d) concentrically embodying the respective outlines of various shapes and sizes of prostheses, - at least one plurality of first holes (6a, 6b, 6c, 6d, 6e, 7a, 7b, 7c, 7d, 7e) arranged along the transverse line passing through the centre of the said sheet, and - at least one plurality of second holes (8a, 8b, 8c, 8d, 8e, 8f, 8g, 9a, 9b, 9c, 9d, 9e, 9f, 9g) arranged along the longitudinal line passing through the centre of the said sheet, - at least one plurality of graphical representations (3a, 3b, 4a, 4b, 5a, 5b, 5c, 5d) concentrically embodying the respective outlines of various shapes and sizes of prostheses, - at least one plurality of first holes (6a, 6b, 6c, 6d, 6e, 7a, 7b, 7c, 7d, 7e) arranged along the transverse line passing through the centre of the said sheet, and - at least one plurality of second holes (8a, 8b, 8c, 8d, 8e, 8f, 8g, 9a, 9b, 9c, 9d, 9e, 9f, 9g) arranged along the longitudinal line passing through the centre of the said sheet, - ii) the device is applied to the external face of the region of the skin of the patient, for example under which the hernia defect is located, and the centre of the sheet of material of the device is substantially aligned on the site of the hernia defect, - iii) the sheet of material is pivoted so as to select the graphical representation corresponding to a size and shape of a prosthesis constituting the best coverage of the hernia defect, iv) using the first and second holes of the device, the said skin is marked in the transverse direction of the sheet and in the longitudinal direction of the sheet.