Why Window line is a form of open-wire (ladder) feed line where two parallel conductors are held apart by a plastic web with rectangular windows cut out of it. The wide conductor spacing and mostly-air dielectric give it a high characteristic impedance, nominally 450 ohms. That wide spacing and low dielectric loss also make it much lower loss than coax, which is why it is popular for multiband doublets fed through a tuner.
Watch out 50 and 75 ohms are the standard coaxial cable impedances (50 for transmitters, 75 for TV and hardline); 300 ohms is the familiar twin-lead TV feed line, but window line with its cut-out windows runs higher at 450 ohms.
Ladder/window line = 450 ohms. Twin-lead = 300. Coax = 50 or 75. Wider spacing, higher impedance.