The angle of your seat
If your seat (whether that’s a chair, bench, or cushions) is flat, then this will cause you to slump, with all of the problems that follow from slumping.
Your seat should have a slight forward angle, to allow you to be able to sit upright with no effort. As explained earlier, you can achieve a good angle when sitting on a chair by having a 1″ (2.5cm) wooden block under each of the back legs. Even books can be used.
If your chair or stool is too steeply angled, however, then this will tend to throw your weight forward and cause over-arching.
This principle also applies when sitting on cushions, although of course you won’t want to put wooden blocks or books under your zafu. If you sit towards the back of your cushion, your pelvis will tilt backwards, and you will end up slumping. In order to have a slight forward tilt to your cushion, your weight needs to be to the front of the cushion.
It’s ideal if you can actually try out a stool for a while. Once you’ve done a few 30-minute meditations on a stool, you have a good idea whether it’s right for you. However, I must say that if you do an intensive meditation retreat (by which I mean at least seven to eight hours of meditation per day) a stool that you thought was comfortable under less demanding circumstances may reveal its limitations.
An excellent solution to help you find the right angle on your seat is to use a meditation stool with rounded legs, as in the one illustrated. When you sit on such a stool it automatically adjusts to the right angle for your body size, although you may still have to play around with cushions between your bottom and the stool in order to get the right height.
An even better solution, although a more expensive one, is an adjustable-height meditation stool. These have adjustable feet at the front and back of each side of the stool. You can therefore adjust both the height and tilt of your seat. The feet lock in place using thumb-nuts, and no tools are required to make the adjustment. This kind of stool is what I use for my daily practice and it’s saved me from a lot of discomfort on intensive retreats. I can’t praise this stool highly enough.
You can make a basic meditation stool using these instructions (PDF format).