The big three networks all had their starts in the world of radio. Both ABC & NBC had logos that reflected their beginnings:
Of the four major networks, NBC has changed its logo the most. As the network that has suffered the biggest lows (as well as several highs) it apparently has seen rebrandings as a possible solution to its problems.
CBS, on the other hand, has probably changed its logo the least. Its current eye logo was introduced right after this one:
Like CBS, Fox has also barely changed its logo, though it has been around the shortest amount of time.
