What is correction ?
Corrections are mainly used to form the sound of individual tracks in the mix or instruments on the stage in such a way that they harmonize with each other. For example, we can reduce the bass frequencies of the bass guitar to emphasize the percussion foot. With the equalizer we can also amplify the individual frequencies. For example, by raising a 150Hz-300Hz band, the snare drum will sound more aggressive. Another example may be the frequency raise at 15kHz-18kHz, which is responsible for the openness of the vocals.
Correction rules
There are many types of equalizers and they all reinforce or reduce specific frequency bands. The band can be divided into several contractual parts such as high band, low band, etc. This allows you to apply unique settings within each band. Correctors generally allow for individual control over several frequency ranges. In other words, the equalizer consists of a series of single filters, responsible for controlling the individual bands. To understand the basics of the equalizer, we will first discuss the parameters we can manipulate in detail.
Parametric equalizers allow for the most precise control of each band. The bell-like shape is most common in all types of equalizers, allowing you to control three main variables: amplitude, base frequency, and width.
In the pictur below we see The Cubase 8 Pro equalizer , but the setting rules are very similar for all parametric equalizers.

Frequency with center at 185Hz has been reduced by 6dB.
Each single equalizer filter has three basic parameters:
Frequency - this is the center frequency that is placed in the band. In the image below, the center frequency is 4900Hz.
Reinforcement - ie the value of raise or reduction. The example below shows a 5dB raise.
Q - parameter responsible for the frequency range around the center frequency. The high value of this parameter will result in a narrow range, while a low wide one. In the image below, the Q parameter is 1.4, resulting in a relatively wide range. We can see on which neighboring frequencies will have affect the corector

Apart from the classic type of bell-shaped equalizer, there are also other types of filters.
Lower / Upper Filter - This is a one-sided filter. By selecting the base frequency, we reduce all frequencies below (high pass filter) or below (low pass filter). We can also determine how rapidly this reduction occurs. Usually, we have a choice of multiples of 6dB per octave such as 6dB, 12dB, 18dB, 24dB, and so on.
The image below shows a top-pass filter with a base frequency set at 100Hz and a Q parameter of 48dB.

Shelf corrector - this type of filter is also one-sided, and its name is derived from the shape it presents on the graph. Shelf corrector affects all frequencies below (lower band) or above (upper band) of the base frequency. It is less precise than the bell filter, but it is used in other situations.

The above picture illustrates a high-end filter with a center frequency of 3981Hz raised by 2dB. It is worth noting that the filter rises above this value and adds 3.5dB at the upper end of the band.
Summary
Although this guide is based on the studio equalizers used most often when mixing music, it may be the general basis for the equalization theme. It does not matter whether we are dealing with an equalizer for guitar, violin or vocals - the rules are always the same. If we understand what it is, we will be able to more clearly shape the sound of our instruments, and consequently the sound of our entire band.