jan 1, 1960 - By the 1960s, overhead projectors were commonly used in schools.
Description:
By mailer_diablo (Self-taken (Unmodified)) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia CommonsAdded to timeline:
Date:
jan 1, 1960
Now
~ 64 years ago
