Ma-ai refers to the critical distance between opponents, a point at which forces are essentially neutral, but where anything can happen. Fundamental to ma-ai is "ma," roughly defined as the way something (or someone) moves through space over time. Many teachers have stated that ma "cannot be taught," either one has this sense of timing, or one does not. However, ma can be enhanced and developed through training. An iaidoka (a student of iaido) who has a good, well-developed sense of ma has an uncanny sense of time and distance. Combined with a sense of zanshin, it is the difference between a merely competent practitioner and a great one. As in other traditional martial art forms, the ma-ai of iaido embodies the concept of the sphere of protection, but in this case the circle is extended by the use of the sword.