What are the basic elements and rules of block diagram algebra?

Block diagrams represent systems using: blocks (transfer function), summing junctions (add/subtract signals), and pickoff points (signal branching). Reduction rules include: series blocks multiply, parallel blocks add, feedback loop TF = G/(1+GH), moving blocks past summing junction or pickoff requires compensation. Block diagram reduction simplifies complex systems to single transfer function. Mason's gain formula provides direct solution for complex diagrams without iterative reduction.