Picture this: You're a kyu belt in a Chito-ryu dojo, and one day you decide to take an extended trip (go to college) to another city (half-way across the country). You really want to continue your martial arts training, but are unable to locate a Chito-ryu school in the area, so you evaluate the many schools available to you from the intensive research you did (the Yellow Pages) and pick a new dojo based on many important criteria; price (you are a student after all).
Shortly after joining, you're at a work-out, and a senior student in the club has offered to help you learn a new kata.
Sempei: "Zenkutsu-dachi! Did your old school not teach you zenkutsu-dachi? Couldn't have been much of a school!" *
You drop into seisan-dachi, and get a strange look from the sempei.
Sempei: "Hangetsu-dachi is not zenkutsu-dachi. Also, that's a terrible hangetsu-dachi."
You: "What am I doing wrong with this stance? Can you give me a hand with hangetsu-dachi?"
Sempei corrects, fine-tunes and corrects again.
You: "Is this stance used in some kata?"
Sempei: "You've never heard of Hangetsu? Are we going to have to re-teach you everything?" *
You: "Apparently so. Can we go over Hangetsu?"
Sempei: "You're not ready for it, so I'll just show you."
Sempei performs Hangetsu.
You: "Hey, that actually looks a lot like Seisan."
DING! And suddenly, the world makes a bit more sense.
OK. I admit it. That story was about me.
Let's fast-forward twenty-ish years. I'm back at a Chito-ryu school (actually, the same one I moved away from), and I am reviewing all of the kata I learned many (many) years ago with Sensei. We get to Seisan, and my mind goes back to Hangetsu, and I think "if Hangetsu-dachi originated from Hangetsu, then Seisan-dachi must have originated from Seisan. Since Seisan-dachi is not as hard on the body as Zenkutsu-dachi, and offers smoother transitions, that must be the reason why it is the primary stance in Chito-ryu katas.".
DING! And suddenly, the world makes a even more sense.
Now, seisan-dachi has gone through some refinement in the past several years, and no longer resembles hangetsu-dachi as closely as it used to. Modern seisan-dachi is more of a cross between hangetsu-dachi and zenkutsu-dachi. Let's take a closer look.
Width: three fists or shoulder width on inside of feet
Depth: two full foot spans from toe of back foot to heel of front foot.
Weight: 60% front, 40% back
Many styles use zenkutsu-dachi, with only slight variations seen between those styles. Zenkutsu-dachi is a deep forward stance where the back leg is straight, and the front leg is bent at the knee at approximately 45 degrees. Some styles prefer the front foot to be turned slightly inward, some styles prefer the center line of the foot to be pointing straight ahead, and some styles want the inside edge of the front foot to be straight. The key for this stance is the front shin should be perpendicular to the ground. In order to grip the ground better, shime (inward tension) is employed on the front leg. Shotokan turns the back foot almost 90 degrees outwards to make it easier to deepen the stance.
Width: shoulder width on inside of feet
Depth: one full foot span from toe of back foot to heel of front foot.
Weight: 50% front, 50% back
Hangetsu-dachi (Half Moon stance) gets its name from the Shotokan kata Hangetsu, which, in case you are unaware, is the "Shotokan-ized" Seisan (Shuri, not Naha). Hangetsu-dachi is a longer sanchin-dachi, but the back foot is turned towards the front, rather than turned inward. The weight distribution of hangetsu-dachi is 50% (just like sanchin-dachi), and the stance employs shime (inward tension) and shibori (twisting) on both the front and back legs.
Width: shoulder width on inside of feet
Depth: one full foot span from toe of back foot to heel of front foot.Weight: 60% front, 40% back
Seisan-dachi is one of the signature stances used in Chito-ryu, and is derived from hangetsu-dachi, and was actually named hangetsu-dachi before Soke Sensei renamed it to seisan-dachi several years ago (if anyone knows the approximate date, please let me know). Like hangetsu-dachi, shime and shibori are employed on both the front and back legs to solidify the stance. The key difference is that the weight is more forward than hangetsu-dachi, with approximately 60% of the weight on the front foot, and the outside edge of the front foot is pointing straight ahead. The back leg is not straight like zenkutsu-dachi, but rather has a slight bend to it. In order to ensure you are in a proper seisan-dachi, sink into the stance, and then look down at your knee. You should be able to just see the tip of the toenail on your big toe over your knee. That is the perfect angle for your stance, and from there, the angle of our knee will not change when transitioning to shiko-dachi, kosa-dachi, uchi-hachiji-dachi or niko-ashi-dachi. This ensures that there is little to no vertical head movement during a transition. There are other techniques to ensuring a solid seisan-dachi. I'll leave that as a conversation you should have with your Sensei.
I find that a lot of karatekas (especially younger ones) pay far too little attention to their stances. I know that I have suffered from that in the past (OK, recent past), but if you pay attention to the stance and ensure the stance is correct (shime, shibori, other stuff you should already know, etc), after a while, sinking into a correct stance comes natural, and you just need to worry about your technique. The added bonus is, with a solid stance, the technique is stronger as well.
For a little bit of history about stances in Chito-ryu, I recommend reading this Ryushu Newsletter from the Ryusei Karate-do website, where I learned some of the above material.
* The attitude that most of the students had towards other schools and styles was one of the primary reasons why I left the dojo (and martial arts for twenty years), but that's another story.