There are many different grades of plywood including: A, B, C, D, CDX plywood, T and G plywood. It is very important to know what kind of plywood you will need for your job. When looking at the grade there will be two letters which represent the front and back of the plywood.
The highest quality plywood you can buy is A or B, since it will have no holes or cracks. C is about your average grade plywood, and D can have many holes in it. The X in CDX plywood means that is exposure quality, although the D side may have holes and cracks in it. T and G plywood has groves on the edges so they can be interlocked together.
All plywood comes in thicknesses of three, five, seven, or nine plies. The thicker plies are common for A-C and CDX plywood since they are used in construction. Five ply is common for sub flooring, while the lower plies are often used in furniture construction.
When To Use CDX Plywood
CDX plywood is an extremely durable wood, but it is not meant to withstand water damage. It is best used in flooring because it is so strong, light, and will not warp.