Step 4 of 18 of the SMILES tutorial. A bond symbol written between two atoms overrides the default single bond: = is a double bond and # is a triple bond. C=CC#N is acrylonitrile, with a C=C at one end and a C≡N at the other. Bond symbols work inside a branch too, which is how every carbonyl is written — CC(=O)C is acetone. Try O=C=O for carbon dioxide, or C#C for acetylene.
It opens on C=CC#N, which you can edit in place: the structure follows every keystroke, and so does the notation when you draw instead.
Convert between chemical structures and SMILES in both directions, search a list of structures by substructure or SMARTS, and learn the notation step by step. Everything runs in your browser — no structure you open ever leaves your machine — so the tool needs JavaScript.