Click to enlarge. Via Histmag.org.

The crisscrossing lines may look a little confusing at first, but this flight schedule/map from Polish Airlines LOT conveys all the information you need to know to fly around Europe in 1939. The thick lines are Polish Airlines LOT routes, while thinner lines represent partner companies or connecting flights. Transit Maps explains how to read the rest:

Red lines show daily service, black lines show weekday or irregular service. The departure and arrival times for each flight are indicated within the circles at each city – for the most part, the bigger the circle, the more flights serve it, although Warsaw’s importance is overstated somewhat as LOT’s main hub. Red times show the different schedules for Sundays. The flight/route number for each flight is superimposed over each route line

The infographic isn't just interesting for what it shows, but also for what one note at the bottom says:

REMARK: The services on all lines passing through PRAHA are suspended until a special notice is given.

Prague (PRAHA), along with the rest of Czechoslovakia, was under Nazi occupation already, an ominous preview of what would soon come for much of Europe.