The end lines and sidelines make up the football boundaries.
You can think of anything inside of the rectangle in the picture above as inbounds. Everything on or outside of this rectangle is out of bounds.
A player is inbounds if he is within the football boundaries (sidelines and end lines).
Football Out of Bounds
A player is out of bounds if he is on or outside the football boundaries. A player is always either inbounds or out of bounds in football.
Football Boundaries Rules
The rules of the football boundaries work like this:
Touching The Boundaries
If a player touches any part of the boundaries (sidelines and end lines) or crosses the football boundaries in any way he is considered out of bounds.
Football Complete Pass
In college football (NCAA football) and high school football, the catching rule states that a player must have at least one foot inbounds when catching a pass.
In the NFL, a player must have both feet inbounds when catching a pass for. Otherwise, the pass is ruled incomplete.
KEY TAKEAWAY: one foot must be inbounds in high school and college football (NCAA football). Two feet must be inbounds in the NFL.