Re-order paragraphs

b) Piano keys are generally made of spruce or basswood, for lightness.

c) Spruce is normally used in high-quality pianos.

a) Traditionally, the black keys were made from ebony and the white keys were covered with strips of ivory, but since ivory-yielding species are now endangered and protected by treaty, plastics are now almost exclusively used.

d) Also, ivory tends to chip more easily than plastic.