Two or more lines are said to be concurrent if they intersect in a single point. Two lines concur if their trilinear coordinates satisfy
(1)

Three lines concur if their trilinear coordinates satisfy
(2)
 
(3)
 
(4)

in which case the point is
(5)

Three lines
(6)
 
(7)
 
(8)

are concurrent if their coefficients satisfy
(9)
