Recently we wrote that the bells -- we called them a "carillon" -- in the tower of the Old Post Office Building at 12th Street and Pennsylvania Avenue are played on national holidays and other special occasions.

That's true, but if you want to hear the ringing at other times, go down to the location 'most any Tuesday evening from 7 to 9:30. That's when members of the Washington Ringing Society, who provide the man- and womanpower for its bells and those at the Washington Cathedral, do their weekly practice. The captain in charge is ordinarily Cecily W. Rock.

Rock and John W. King, captain of the cathedral bells, pointed out in a letter that the 10 Post Office bells, a bicentennial gift from Britain, aren't a carillon.

"A carillon is a keyboard instrument by which stationary bells are chimed by pulling the clapper against the side of the bell (and it can thus play tunes)," they wrote. "Change ringing bells [the technical name for those in the tower] are swung through a 360-degree arc by pulling ropes attached to them, and involves mathematical permutations, not tunes. Because of their weight, they cannot be rung quickly enough to play tunes."

The Washington Cathedral, they concluded, has the only tower in the world with both a carillon and change ringing bells.