Code points may range from 0 to 0x10FFFF (= 1 114 111). We divide this range into 17 planes, which we number from 0 to 16. Of these 17 planes, only 6 are currently "populated" (see Fig. 2-2):
Plane 0, or the BMP (Basic Multilingual Plane), corresponds to the first 16 bits of Unicode. It covers most modern writing systems.
Plane 1, or the SMP (Supplementary Multilingual Plane), covers certain historic writing systems as well as various systems of notation, such as Western and Byzantine musical notation, mathematical symbols, etc.
Plane 2, or the SIP (Supplementary Ideographic Plane), is the catchall for the new ideographs that are added every year. We can predict that when this plane is filled up we will proceed to Plane 3 and beyond. We shall discuss the special characteristics of ideographic writing systems in Chapter 4.
Plane 14, or the SSP (Supplementary Special-Purpose Plane), is in some senses a quarantine area. In it are placed all the questionable characters that are meant to be isolated as much as possible from the "sound" characters in the hope that users will not notice them. Among those are the "language tag" characters, a Unicode device for indicating the current language that has come under heavy criticism by those, the author among them, who believe that markup is the province of higher-level languages such as XML.
Planes 15 and 16 are Unicode's gift to the industry: they are private use areas, and everyone is free ...