A pair of emery files each with 2 different filing surfaces suitable for normal to strong nails.
A professional length, double-sided nail file with 2 surfaces. A 240 grit, fine filing surface and a coarser, 180 grit surface.
The 240 surface is suitable for normal nails whilst the coarser, 180 surface is suitable for normal to strong nails.
The professional technique for using a trditional emery board nail file is to hold the file at one end to allow for one long, continuous sweep along the nail edge.
Hold the file at one side of the nail then sweep in one movement towards the centre of the nail tip. Repeat the same movement from the other side of the nail, again sweeping towards the centre.
This reduces the length fast and leaves the free edge smooth.