Kendo, through its origin lay in fighting by use of the real sword, nowadays is played between two contestants with bamboo sticks (Shinai) whereas Iai is played singly with either the real sword or an imitation one.
Iai is an art of self defence that you draw the sword immediately against an enemy's sudden attack to win at the very moment when the sword leaves the sheath. It was initiated about four hundred years ago and has been followed by many kendo players ever since.
Its training and practice is very hard; because the purpose of Iai is to ultimately master the secret to win over your enemy without drawing the sword, or to spiritually conquer your opponent with your sword left in the sheath; that is you may settle problems without resorting to arms.
Iai was initiated as a martial art to win at the very moment when the sword leaves the sheath, taking a posture to respond promptly against the enemy's sudden attack.
Iai-Do and Kendo are just like the two wheels of a car, being so closely related and having been lasting together through the long history of Japan's martial arts.
Zen Nihon Kendo Renmei (All Japan Kendo Federation), Zen Ken Ren, or ZNKR as its abbreviation lately developed a new program for practice called Zen Ken Ren Seitei Iai (ZNKR's enacted Iai instructions).
It reflects various old schools of Iai-Do in that their representative or basic forms, techniques, manners, etc. have been adopted in this new program. It is highly useful, therefore, to both beginners and experts.
Zen Ken Ren Seitei Iai is mostly applied to Iai Dan examinations and Iai matches that are operated by ZNKR.