A disc with the same thickness as a CD, that is, 1.2mm, isn't suitable for error-free reading of high-density data. It's too thick. Due to the impact of man ufacturing processes and usage, a disc's surfaces may not be perfectly level. Even a slight distortion can deteriorate laser focus. The higher the data density, the greater the risk of error when data is read.

And, the thinner the disc, the less is the impact of distortion on focus deterioration. But adoption of a thin disk makes it difficult to achieve CD compatibility. The solution: the DVD consists of two discs, each 0.6mm thick, bonded back to back to achieve strength and the same 1.2mm thickness as a CD. Also, compared with a single 1.2mm-thick disc, the DVD configuration in which two discs are bonded is less prone to warping due to heat or humidity. All external dimensions of the DVD are, of course, the same as those of a CD. Moreover, both sides can be used.