If a line makes an angle θ with x - axis in the positive direction, then θ is called inclination of the line and 'tan θ' is called slope of the line and is denoted by m.
* Slope of x - axis or any line parallel to it is "Zero".
* Slope of y - axis or any line parallel to it is "tan 90o" not defined.
* Slope of a line passing through two points
    (x₁, y₁) and (x₂, y₂) is   x₁ ≠ x₂
* Two lines are parallel if their slopes are equal.
* Two lines are perpendicular if product of their slopes is equal to '-1'.
* Equation of x - axis is y = 0
    Equation of y - axis is x = 0
    Equation of line parallel to x - axis is y = K (K = constant).
    Equation of line parallel to y - axis is x = K (K = constant).
Equation of line passing through (x₁, y₁) and having slope 'm' is y - y₁ = m(x - x₁)
* Equation of line passing through (x₁, y₁) and (x₂, y₂) is 
                         (y - y₁) (x₂- x₁) = (y₂ - y₁) (x - x₁).
* Equation of line having slope 'm' and Y - intercept 'c' is y = mx + c.

* Equation of line making X - intercept 'a' and y-intercept 'b' is 
* ax + by + c = 0 is the general form of equation of a straight line with slope  and intercepts ,   respectively on coordinate axes.
* Equation of line with inclination θ and passing through (x₁, y₁) is 
             (symmetric form) where θ  (0, Π/2) U (Π/2, Π)
* x = x₁ + r cos θ, y = y₁ + r sin θ are the parametric equations of line passing through (x₁, y₁).