Ma-ai (間合い) which is translated as "Distance" or "Interval" is a very important concept in Japanese martial arts. Below is an article from the Aikido Journal on this topic. If you click on the title of this post, you'll be directed to the whole article. Enjoy.
by Diane Skoss
My cousin, who runs a karate school in San Jose, California, says that the one who controls the distance in an encounter is the one who controls the situation. One of the shihan of the Japan Aikido Association, when asked about how, using aikido, to deal with a karate practitioner, replied simply, "Maai."
We've all heard similar statements and all have been admonished during training to be aware of the maai, often translated as combative engagement distance, but perhaps more accurately rendered "combative interval." When I first heard the word in a Tomiki aikido dojo in the U.S., I thought it referred to a simple spatial relationship-the distance at which I could, in a single movement, reach an opponent with my attack. Conversely, I also discovered, it was the distance at which an attacker could reach me!
What I didn't quite get at first was the extent to which this was not one, but two, sometimes vastly different, distances. When my then training partner, Meik Skoss, casually remarked, over coffee and donuts after jukendo (bayonet Way) training one morning, "Of course, you know that my maai in relation to you, will always be different from yours to me--even though the distance between us is constant," I nodded, and pretended to have the foggiest notion of what he was talking about. It became clearer soon after when I ran into my friend Bill, who is over six feet tall, in the company of his girlfriend, who is five foot nothing. If the two of them were to stand side-by-side facing me, at (Bill's) arms length away, I would be fully within Bill's maai, and just outside of his girlfriend's. They would both be in my maai. If Bill took one step back, he might very well be out of my maai, yet I would still be within his. These differences are naturally based on the length of each individual's arms and legs. Two more elements, speed and timing (hyoshi) can also affect the effective combative interval. What it all adds up to, is judging the constantly changing maai, different for each individual and each type of attack, is incredibly complicated. And of course, our teachers tell us, we must learn to make this evaluation virtually subconsciously and instantaneously.