Drawing is a manifestation of symbolic activity of man, that is, its ability to represent things by means of signs which constitute the substitutes of those things. For its nature, it’s intended to convey meanings among individuals who recognize it, it is therefore a means of communication. In this context, the project drawing has a place which is a means of communication by which information is transmitted through the reduction of the concept in concrete and legible signs. These signs semantically allow the reading and the perception of the orderly content that constitutes the signifier; such signs represent the elementary cell of the graph as binary action among two abstract classes of concepts: the meaning and the signifier. The first one (the meaning) is the mental image that is produced it, depends on the experience and is induced by the second (the signifier) which is the physical vehicle, concretely materialized with an ИСТОРИЯ НАУКИ И ТЕХНИКИ. БИОГРАФИИ In the past, the representation of design profited from some symbol-forms such «architectural orders» or building types, which were common and certain patterns, but with the changes of the nineteenth century, it has been assuming ever greater autonomy and symbolic-expressive capacities. The representation, endowed with a very articulated syntax, results fit to rather reproduce complex representations, such as the graphs of assemblage of metallic works or preparation for theirs single components, but also the diagrams for the temporal planning of the operations of yard. In the interpretation of the drawing, therefore, interesting to define the relationship between the content of the information and the characteristics of the communication medium. The versatile design allows you to articulate it in multiple uses, each expressed with a specific pattern; in the nineteenth century has gone by defining the division of the models into three classes, for the purposes of representation, which has largely remained unchanged up to us: the iconic models that communicate the concept trying to maintain the qualitative and quantitative aspects, the logic models that reproduce the phenomenon through the use of symbols and relations between them and the verbal patterns that represent the phenomenon by describing lexical character and properties. The latter have a dual nature, of instructions or information. Information go to complete the information of the representation with denominations, attributions of materials, dimensions, explanation of the characteristics of the surfaces of the conservation status. Instructions, serve to explain the context in which you put the subject of the representation with formal definitions and descriptions. It is easy to imagine that the oral model doesn´t exhaust him in a simple terminological description, but consists mainly of numerical specifications to a graphic support which they refer. In the drawing of project it doesn’t use a single model, but it is assisted to their mutual integration, with the prominence of one in comparison to the others according to the object represented of the period and of the professional sector which refers. In the given plans it’s evident the heterogeneity of the employed style; the different kind of the object does not affect the model: the bulkhead of a rail yard has markimage. The system of signs can be organized in a code when it allows the recipient an unambiguous interpretation of the written message. It becomes, therefore, necessary to determine the encoding used in the act of writing the graphics, in order to ensure an exact decoding, avoiding any possible ambiguity in the subsequent phase of reading.