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.
In organic chemistry sigma bonds (Ï) are always the first bond between two atoms, resulting from overlap along the bonding axis (of hybrid orbitals), while pi bonds (Ï) are second and third bonds resulting from the overlap of p orbitals, above and below (or in front and back of) the bonding axis.