Tattoo machines are not limited to just these types. A common variant is having a "cutback", which uses stiffer front springs. This is more commonly used in liners, but is known to be used on shader machines, more typically for portrait work. Machines are usually categorized into long stroke and short stroke varieties. The longer-stroked machines are good for coloring and shading, as well as sculpting lines, while doing less damage to clients' skin. Shorter-stroke machines are commonly used for lining in a single pass style, and also in a shader setup to achieve a more subtle gradation of black such as would be found in portraits. Length, width, tension, angle, and stiffness of the spring varies the functionality of the machine. The contact gaps, as well as capacitors and even the style of machine and its angles of deflection, can also all be variants in machine tuning. The proper tuning of the machine is essential for the type of machine being used, also for the type of tattoo the artist is doing.