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.
By Anne Marie Helmenstine, Ph.D. Definition: Sigma bonds are covalent bonds formed by direct overlapping between two adjacent atom's outermost orbitals. The single electrons from each atom's orbital combine to form an electron pair creating the sigma bond. Sigma bonds are generally denoted by the Greek letter Ï.