This study dealt with one of the issues of phonetic research in the book “Sharh Al-Mufassal fi San’at Al-A’rab Al-Mosom by Al-Takhmir “by Al-Qasim bin Al- which is the phenomenon of Slanting for which Al-Khwarizmi singled out an entire chapter.Slanting is one of the phenomena that have reached us through the generations with its different degrees and types. It has had great interest from Arabic and Tajweed scholars, and Al-Khwarizmi is considered one of the scholars who gave it that interest. From Al-Khwarizmi's views and directives, then I showed the positions of the ancient scholars, then to stand on the data of the modern phonetic lesson. The study concluded with several results, including: Al-Khwarizmi acknowledged the necessity of Slanting. In addition, Al-Khwarizmi's style was characterized by ease and unobtrusive brevity in dealing with phonetic issues.