Two rows are better than one! The original idea was to supplement the home row (tone) with a companion row (fourth) to facilitate playing in both directions and to add a second key. The most common keys are C-F, G-C, D-G, A-D, and Bb-Eb. Another clever way to use two rows is to separate the keys by a semi-tone (half step) to create a fully chromatic (yet bi-sonoric) instrument. The B-C and C#-D layouts are hugely popular in Irish traditional music with many ornaments and rolls adapted from the unique layout of these instruments.