Musical genres and forms differentiate one another in their concepts and characteristics, but they are also interrelated. Musical forms may be helpful by describing the feature of a musical genre, and musical genres can provide musical forms the needed works from various epochs and types for concrete verifying the theories of forms. The theoretical character of musical form is strong, hence it's apt to neglect the historical background and the status of development of times, therefore inadequate or even biased. Some music like canon, fugue and variations are either musical forms or genres, they must be clarified in understanding of cognitive. Between musical form and musical genre exists a notable level of formal structure, it's the architectonics. To establish the concept of architectonics in music theory is the main focus of this article, including its meaning, characteristics, selection, change and constancy, and a preliminary study about category. In conclusion, the two statements about the form of the first movement of concerto in the classical period are especially cited as examples for interpretation, that the consideration of characteristics of musical genres for finding out the correct and rational view of a musical form is proper and worthy.