require, specification of stereoisomers. Configuration around double bonds is specified using the characters / and \ to show directional single bonds adjacent to a double bond. For example, F/C=C/F (see depiction) is one representation of trans-1,2-difluoroethylene, in which the fluorine atoms are on opposite sides of the double bond (as shown in the figure), whereas F/C=C\F (see depiction) is one possible representation of cis-1,2-difluoroethylene, in which the fluorines are on the same side of the double bond. Bond direction symbols always come in groups of at least two, of which the first is arbitrary. That is, F\C=C\F is the same as F/C=C/F. When alternating single-double bonds are present, the groups are larger than two, with the middle directional symbols being adjacent to two double bonds. For example, the common form of (2,4)-hexadiene is written C/C=C/C=C/C. As a more complex example, beta-carotene has a very long backbone of alternating single and double bonds, which may be written CC1CCC/C(C)=C1/C=C/C(C)=C/C=C/C(C)=C/C=C/C=C(C)/C=C/C=C(C)/C=C/C2=C(C)/CCCC2(C)C. Configuration at tetrahedral carbon is specified by @ or @@. Consider the four bonds in the order in which they appear, left to right, in the SMILES form. Looking toward the central carbon from the perspective of the first bond, the other three are either clockwise or counter-clockwise. These cases are indicated with @@ and @, respectively (because the @ symbol itself is a counter-clockwise spiral). For example, consider the amino acid alanine. One of its SMILES forms is NC(C)C(=O)O, more fully written as N[CH](C)C(=O)O. L-Alanine, the more common enantiomer, is written as N[C@@H](C)C(=O)O (see depiction). Looking from the nitrogen–carbon bond, the hydrogen (H), methyl (C), and carboxylate (C(=O)O) groups appear clockwise. D-Alanine can be written as N[C@H](C)C(=O)O (see depiction). While the order in which branches are specified in SMILES is normally unimportant, in this case it matters; swapping any two groups requires reversing the chirality indicator. If the branches are reversed so alanine is written as NC(C(=O)O)C, then the configuration also reverses; L-alanine is written as N[C@H](C(=O)O)C (see depiction). Other ways of writing it include C[C@H](N)C(=O)O, OC(=O)[C@@H](N)C and OC(=O)[C@H](C)N. Normally, the first of the four bonds appears to the left of the carbon atom, but if the SMILES is written beginning with the chiral carbon, such as C(C)(N)C(=O)O, then all four are to the right, but the first to appear (the [CH] bond in this case) is used as the reference to order the following three: L-alanine may also be written [C@@H](C)(N)C(=O)O.