A trend of 7 or more consecutively plotted points in either an upward or downward direction indicates the potential presence of a special cause in the process.

UCL = Upper Control Limit
LCL = Lower Control Limit
x-double bar = Mean (average of all the x-bar values (subgroup averages))

Note: 2 consecutive plots of the same value are considered part of the same trend.

Trends can indicate process change resulting from many things, for example:

A downward trend indicates an improvement on Range, Standard Deviation, and Attribute charts.