Linear inequalities in two variables (ax + by < c) represent regions in coordinate plane. Solution set includes all points (x,y) satisfying inequality. Graphing: (1) Replace inequality with equality (ax + by = c) to get boundary line; (2) Find intercepts; (3) Draw line (dashed if < or >, solid if ≤ or ≥); (4) Test point to determine shaded region. Example: x + y < 4. Boundary: x + y = 4. Test (0,0): 0 + 0 < 4 (true), so shade region containing origin. System: Multiple inequalities; solution is intersection of all regions. Exam tip: Shade correctly (test point method). Practice: Identify feasible region for optimization problems.