How many minutes before sunset are Shabbat candles lit?

Different communities have varying customs for candle lighting time: most Ashkenazi communities light 18 minutes before sunset, some light 20-22 minutes before, and in Jerusalem the ancient custom is 40 minutes before sunset. The variation reflects different interpretations of 'tosefet Shabbat' — adding time from the weekday to the holiness of Shabbat. Whatever the custom, the candles must be lit BEFORE sunset, as kindling fire is one of the 39 prohibited labors on Shabbat.