what is the not symbol on tech tree

[bill_d]

Why the not symbol (circle with diagonal slash) is showing up on the tech tree next to some techs? Is it a hint?

I've play civ3 for about 15 hours and this site is fantastic at picking up where the manual ends. Thx.

 

[Hooray]

It means that it is a not a required technology. In other words, you don't have to research that tech in order to reach the next era.

 

[elementgoo]

Right on, Hooray!