like cylinder records, the sound in a diamond disc's groove was recorded by the vertical method, as variations in the depth of the groove cut. at that time, with the notable exception of pathé records, which used yet another incompatible format, a disc's groove was normally of constant depth and modulated laterally, side-to-side. the vertical format demanded a perfectly flat surface for best results, so edison made his diamond discs almost one-quarter of an inch (6 mm) thick. they consisted of a thin coating of a phenolic resin virtually identical to bakelite on a core of compressed wood flour, later also china clay, lampblack for color, all in a rabbit-hide glue binder. with very rare exceptions, all were about ten inches in diameter, but they used a finer groove pitch (150 threads per inch, or "tpi") and could play longer than lateral ten-inch records—up to minutes per side.