Since skeletal structures were introduced in the latter half of the 19th century, their appearance has undergone considerable evolution. The graphical conventions in use today date to the 1980s. Thanks to the adoption of the ChemDraw software package as a "de facto" industry standard (by American Chemical Society, Royal Society of Chemistry, and Gesellschaft Deutscher Chemiker publications, for instance), these conventions have been nearly universal in the chemical literature since the late 1990s. A few minor conventional variations, especially with respect to the use of stereobonds, continue to exist as a result of differing US and UK and Continental European practice, or as a matter of personal preference. As another minor variation between authors, formal charges can be shown with the plus or minus sign in a circle or without the circle. The set of conventions that are followed by most authors is given below, along with illustrative examples.(1) Bonds between sp2 and/or sp3 hybridized carbon or heteroatoms are conventionally represented using 120° angles whenever possible, with the longest chain of atoms following a zigzag pattern unless interrupted by a "cis" double bond. Unless all four substituents are explicit, this is true even when stereochemistry is being depicted using wedged or dashed bonds ("see below").