Body effect
Beginner
A switch gets a little harder to turn on when it sits higher up in a stack of switches.
Novice
The threshold voltage rises when the source is at a higher voltage than the body, which happens in transistors stacked in series.
Expert
. It weakens upper transistors in series stacks and is exploited deliberately by body biasing.
Explained in The switch (Transistors).
See also: Body (substrate or well), Threshold voltage (Vt).