Arabic endeavored to get rid of successive similar letters, due to the fact that they are difficult to pronounce, using multifarious ways; one of which was to substitute some of them for either a consonant or a vowel so as to soften and ease their pronunciation. Substitution phenomena  do not generate adequate resort due to its divergent localities in Arabic. Because of all of this, this study attempts to manifest these phenomena.

            Not only did I pursue and encompass those phenomena, but I also tried, as much as I could, to bring out its causes, display Arab intentions of using them and demonstrate Arab linguists' attitudes towards all of this.