The invention that changed the art world during the 19th century is the paint tube.
Short answer-half note.
In common time the quarter note equals one beat, and there are four quarter notes in a measure.
Songs in alla breve contain two beats per measure, with the half note equaling one beat. That means that the time signature of the alla breve is represented as 2/2 (not 4/4 as common time is).
