Many new instruments not previously thought appropriate for folk music have come into usage during the various revivals and in the last century. As people have already more or less stated above folk instruments generally have traditionally, for obvious reasons, been portable ones and relatively inexpensive. However for certain types of trad music the piano has for at least a century been traditionally used, dance music being the most obvious arena. In the early 19th century in the UK most working class folk aspired to have a piano in the front parlour. For many years it provided the basic music in the pubs before canned music took over and both these were for song accompaniment. I can remember recording singers of sea songs, bawdy songs etc to a piano in a Hull pub, and the first country dance band I played in had a vamping piano dashing out a very danceable rhythm. The last ceili bands I had all relied on keyboards for the main rhythm section and accordion for lead.