If a pair of linear equations is consistent, then the lines will be
Condition for a consistent pair of linear equations -
Consistent equations means the equations must have solution so lines may either coincide(with infinitely many solutions) or with a unique solution {lines must not be parallel}.
1) Must not be parallel so,
a1/a2 
b1/b2 [intersecting lines with unique solution]
2) Also the lines can coincide, so,
a1/a2 = b1/b2 = c1/c2 [coincident or dependent]
3