Yoga blocks come in many shapes and sizes and are made of many materials. Wooden blocks are harder and sturdier, but are often more expensive. If you opt for purchasing a foam block, make sure that it is made of dense foam. The lighter foam blocks often get compressed over time and shrink irregularly, which makes them lose their shape and become next to useless.
Blocks are used in standing poses when you are unable to safely bring your hand to the floor. For this reason, a pair of blocks is often more convenient than a single block, one for each side. Blocks are extremely versatile. They can be used for sitting on, lying back over, resting limbs on and more.
A great alternative to blocks are books. If you have old phone books hanging around, save them and tape them up with packing or gaffer’s tape so they keep their shape as a very inexpensive alternative.
The standard is the wooden block which comes in many different weights of wood, both hollow and solid. Many places online sell them. Custom Yoga Props is an excellent business based in Arizona that make affordable, high quality wooden yoga props, including a sturdy, lightweight wooden block:
This foam block by HuggerMugger is a standard that is also offered by many other companies. It is useful for most purposes, being light and dense. It is not so good for weight-bearing poses such as Setu Bandha (Bridge Pose) with the sacrum on the block as it does compress unevenly while in the pose.
HuggerMugger also do a cork block that is lightweight. It is, however, a smaller block that does disintegrate around the edges over time.