The parameters "A", "B", "C", and "D" differ depending on how the desired model handles the line's resistance ("R"), inductance ("L"), capacitance ("C"), and shunt (parallel, leak) conductance "G". The four main models are the short line approximation, the medium line approximation, the long line approximation (with distributed parameters), and the lossless line. In all models described, a capital letter such as "R" refers to the total quantity summed over the line and a lowercase letter such as "c" refers to the per-unit-length quantity.