Iframe Tag just adds another window on your Webpage and due to which whenever a browser loads a page with an iframe it loads the original Page and Iframe as a separate 2 different pages. So in some cases Iframe is not-supported and thus, displays an Error.
Sigma is a Greek letter that corresponds somewhat to our letter S and thus sigma is often used to stand for some concept that starts with S. There is an upper case (capital) sigma and a lower case (small) sigma. They are and. is most ofter used to indicate a sum.
San was an archaic letter of the Greek alphabet. Its shape was similar to modern M, or to a modern Greek Sigma turned sideways, and it was used as an alternative to Sigma to denote the sound. Unlike Sigma, whose position in the alphabet is between Rho and Tau, San appeared between Pi and Qoppa in alphabetic order.