The normal distribution (also known as the Gaussian distribution) is a continuous probability distribution. Most processes that are 'in control' conform to a normal distribution, or to some variant of it. Normality is also a condition of many statistical tests.
The normal distribution has a 'bell shape':
The proportion of the process output that falls within a range of values is represented by the area of the curve. Values are:
|Between -1 and +1 Standard Deviation
||68.3% (about two thirds)
|Between -2 and +2 Standard Deviation
||95.5% (about 95%)
|Between -3 and +3 Standard Deviation
All values can be obtained from the Normal Distribution Tables.