Originally an Able Seaman was one able to "hand, reef and steer". That is, he could handle sails and ropes, reef sails as required and steer by the compass or the wind. An Ordinary Seaman was a learner who lacked these skills. Able Seamen acquired many skills, especially in rope work. In the days of sail, seamen actually rated themselves. The system worked because a man who signed as an AB risked dire punishment if he turned out to be incompetent.
In 1912 an Able Seaman was one with three years satisfactory experience as an Ordinary Seaman. As Ordinary Seamen did simple tasks like sweeping decks, the standard was low. Today there is formal training for seamen, at least in advanced countries and certificates are awarded.