Figures showing the prior art are usually unnecessary and should be canceled. Ex parte Elliott, 1904 C.D. 103, 109 OG 1337 (Comm’r Pat. 1904). However, where needed to understand applicant’s invention, they may be retained if designated by a legend such as “Prior Art.”
¶ 6.36.01 Illustration of “Prior Art”
Figure  should be designated by a legend such as --Prior Art-- because only that which is old is illustrated. See MPEP § 608.02(g). Corrected drawings in compliance with 37 CFR 1.121(d) are required in reply to the Office action to avoid abandonment of the application. The replacement sheet(s) should be labeled “Replacement Sheet” in the page header (as per 37 CFR 1.84(c) ) so as not to obstruct any portion of the drawing figures. If the changes are not accepted by the examiner, the applicant will be notified and informed of any required corrective action in the next Office action. The objection to the drawings will not be held in abeyance.