In Japanese traditional martial arts, the distance at which you can successfully strike your opponent and thus be struck by them is called issoku itto no ma-ai. Issoku itto no ma-ai means one step, one cut spacing.
Generally speaking in swordsmanship the safest distance between you and your opponent is supposed to be six feet. This distance is called toi ma-ai or far spacing. They cannot strike you and you cannot strike them. Once you move in and are within striking distance the spacing is called chikai ma-ai or close spacing. When one reaches a distance where they are able to strike this distance is called uchi ma-ai or inside spacing.
Controlling the spacing and timing are the keys to victory in not only swordsmanship but in Aikido as well. If we can control the spacing then we will be able to control the timing and therefore if you are controlling the timing then you are already in command of the spacing.
Beginners tend to stand too close (unaware of the attack) or too far (too afraid of the attack) but as they become more experienced they will come to understand what the proper distances are for each attack and technique. An experienced practitioner has learned how to control the spacing to their advantage and uses it to fend off attacks or create openings to inspires attacks.
It is said that in swordsmanship the margin of death is roughly an inch. If you can pass the tip of your opponents blade then you can kill them. Knowing this, the tiniest of movement can be the difference between life and death. One can only control the spacing after they have gained technical mastery of the techniques. So students should take the time to learn the proper spacing for all the techniques That is why the proper ma-ai depends on one's ability.