It is a general requirement that while drafting a provisional application one should ensure that the subject matter to be claimed by a complete application is “enabled” in the provisional application. Otherwise, when you file claims in the complete application that was not enabled in the provisional application, you may not be entitled to the benefits of the priority date established by the provisional application.
Enablement of the subject disclosure can be done either by including specific texts and/or by including drawings. This is particularly applicable for patent applications related to mechanical domain, or when the key aspect of the invention is a design feature, for example, in consumer goods. Patent drawings are the visual form of patent description or invention which aid in understanding the invention clearly. So if a particular feature of an invention is clearly visible in the drawings and could be understood by a person skilled in the art by means of drawings, the drawings alone may be considered sufficient for “enablement” purposes. In other words, if drawings are required to understand any feature of the invention, then it should be included in the provisional application.
The relevant statute that defines this requirement is 35 U.S.C 111(b), which says:
(b) PROVISIONAL APPLICATION.-
(1) AUTHORIZATION. A provisional application for patent shall be made or authorized to be made by the inventor, except as otherwise provided in this title, in writing to the Director. Such application shall include-
(A) a specification as prescribed by the first paragraph of section 112 of this title; and
(B) a drawing as prescribed by section 113 of this title.
So the popular idea that the drawings are not generally required in a provisional patent application is simply not true. Drawings in a provisional patent application are of equal importance as in its corresponding non-provisional or complete application. Also, the requirements of drawings for provisional patent applications are usually the same as that for non-provisional or complete applications. Though, most patent offices do allow the provisional applications with informal drawings. But still the drawings, included with the provisional applications, shall show the inventive feature(s) clearly without any ambiguity.