37 CFR § 1.81 - Drawings required in patent application.
(a) The applicant for a patent is required to furnish a drawing of the invention where necessary for the understanding of the subject matter sought to be patented. Since corrections are the responsibility of the applicant, the original drawing(s) should be retained by the applicant for any necessary future correction.
(b) Drawings may include illustrations which facilitate an understanding of the invention (for example, flow sheets in cases of processes, and diagrammatic views).
(c) Whenever the nature of the subject matter sought to be patented admits of illustration by a drawing without its being necessary for the understanding of the subject matter and the applicant has not furnished such a drawing, the examiner will require its submission within a time period of not less than two months from the date of the sending of a notice thereof.
(d) Drawings submitted after the filing date of the application may not be used to overcome any insufficiency of the specification due to lack of an enabling disclosure or otherwise inadequate disclosure therein, or to supplement the original disclosure thereof for the purpose of interpretation of the scope of any claim.