On most guitars, the strings are not fully parallel: they converge at the nut and diverge at the bridge. Thus, bridge, neck and middle pickups usually have different polepiece spacings on the same guitar. There are several standards on pickup sizes and string spacing between the poles. Spacing is measured either as a distance between 1st to 6th polepieces' centers (this is also called "E-to-E" spacing), or as a distance between adjacent polepieces' centers. Output Some high-output pickups employ very strong magnets, thus creating more flux and thereby more output. This can be detrimental to the final sound because the magnet's pull on the strings (called string capture) can cause problems with intonation as well as damp the strings and reduce sustain. Other high-output pickups have more turns of wire to increase the voltage generated by the string's movement. However, this also increases the pickup's output resistance and impedance, which can affect high frequencies if the pickup is not isolated by a buffer amplifier or a DI unit. Pickup sound The turns of wire in proximity to each other have an equivalent self-capacitance that, when added to any cable capacitance present, resonates with the inductance of the winding. This resonance can accentuate certain frequencies, giving the pickup a characteristic tonal quality. The more turns of wire in the winding, the higher the output voltage but the lower this resonance frequency. The arrangement of parasitic resistances and capacitances in the guitar, cable, and amplifier input, combined with the inductive source impedance inherent in this type of transducer forms a resistively-damped second-order low-pass filter, producing a non-linearity effect not found in piezoelectric or optical transducers. Pickups are usually designed to feed a high input impedance, typically a megohm or more, and a low-impedance load increases attenuation of higher frequencies. Typical maximum frequency of a single-coil pickup is around 5 kHz, with the highest note on a typical guitar fretboard having a fundamental frequency of 1.17 kHz. Humbuckers Single-coil pickups act like a directional antenna and are prone to pick up mains hum—nuisance alternating current electromagnetic interference from electrical power cables, power transformers, fluorescent light ballasts, video monitors or televisions—along with the musical signal. Mains hum consists of a fundamental signal at a nominal 50 or 60 Hz, depending on local current frequency, and usually some harmonic content. To overcome this, the humbucking pickup was invented by Joseph Raymond "Ray" Butts (for Gretsch),