Partnerships need to agree which card to lead from suits headed by A-K. At one time, it was popular to lead the king so that if you led the ace it would not indicate possession of the king. The problem was that if you could lead the king from either A-K or K-Q, partner would not know what signal to give. This issue and a general acceptance of the fact that leading an unsupported ace is rarely best, mean that it is now standard to lead the ace from ace-king (except from A-K doubleton).