X:N N is whatever number you decide to give it. With a single tune in the file it's usually 1.

In ABC2WIN a single file can have at least 199 different tunes, each with its own number (N), so all N's from 1 - 199 are in same ASCII file.

Standard math conventions: - pure integral number is i, j, k, l, m, n, n or caps
variable x, y, z;
fixed constants a, b, c
X, Y, Z are often Cartesian coordinate axes, but x, y, z are sometimes, too.