One of the first things I talk about when it comes to alignment is:

Head over shoulders
Shoulders over hips
Hips over ankles

These three are the basic alignments for standing upright at the beginning of almost all Tai Chi forms and looks something like this (ignore the arm positions):

Image taken from a free Zhan Zhuang course by Water Dragon Arts:

If one of these things is out of alignment then you are leaning forward or backwards, or your posture is out of whack.

It’s easy enough to keep these three alignments in a standing stance, but things can get more complicated in movement, and when you introduce forward and back-weighted stances. At that point I try and keep the following two alignments:

Head over shoulders
Shoulders over hips

The ankles can now be in different places, as reacquired by the stance.

It depends on the style of Tai Chi you do, but if you do a style that advocates a forward lean then you need to make sure that there’s a straight-ish line between your back foot and your head. If you do a style that doesn’t advocate a lean, then your back knee must be bent and you want to keep your body upright with your shoulders vertically over the top of your hips.

But the real answer is that no one style of Tai Chi only does things one way. Most styles contain some moves that lean and some that don’t. Wu style, for example: 

1. 

If you compare late Yang Cheng Fu to early Chen Man Ching postures, you can see that they are very similar, and are both trying to keep the head over the shoulders, over the hips:

While doing the tai chi form, take a moment to think: ‘where is my head in relation to my hips?’ You don’t want to be sticking your bottom out and destroying your alignment, which often happens in transition movements between postures:

A picture paints a thousand words, especially in Tai Chi. I recently found an incredible source of Tai Chi images drawn (I think) by Anthony of Brisbane Tai Chi.

Just scroll down the main page and look at the images – they’re great! Full of tips on alignment and posture for Tai Chi and Zhan Zhuang (standing practice).

One of Anthony’s best images for thinking about how alignment relates to the tai chi form is this one:

I think that image very clearly shows head over shoulder over hip over ankle, and how easy it is to mess that alignment up once you start moving in Tai Chi. You basically want to keep the blocks aligned over each other.

Why?

As it says in the picture, if you align yourself correctly with gravity then your legs become the primary weight holders in the body. That means you can be more relaxed (Sung) in the upper body, so that you can use it to transfer force, instead of tensing up to hold weight that is misaligned. Plus, it just feels better.