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Introduction
Each device has its own hardware, with a motherboard that requires its own bootloader. Therefore, a different procedure is required for each device according to the manufacturer. I will talk below the major smartphone brands, using applications that do not require advanced knowledge and that will make your life easier. For more basic information about Bootloader function, please visit the article below:
Attention: Neither I nor AndroidPIT can be held responsible for damages incurred during proceedings described in this article. We recommend backing up all files on your device before performing the process.
Each method described (with the exception of HTC and Sony devices) requires root permissions enabled on your device. To learn how to get them, check out the articles below:
Samsung
Most Samsung devices already have an unlocked Bootloader. However, there are exceptions, such as the Galaxy S3 connected US carrier Verizon. In the rare case you have one of these devices and want to unlock Bootloader, just download and install the EZ-Unlock app and click the Unlock button.
EZ-Unlock works only for devices manufactured before 2012.
note: EZ-Unlock only works for devices manufactured before 2012. Therefore, if your device does not meet this requirement, it is not compatible and may cause problems. Also, it is possible that the script for your device is up to date, and you will have to create it.
Remember that this app has already caused many problems among Samsung users. I suggest that before using it, you should check to see if other owners have been successful with the process, otherwise use the traditional Bootloader unlocking system via SDK.

Galaxy S3 a rare exception among Samsung bootloader locked / AndroidPIT
Nexus
If you're a happy Nexus 5 or Nexus 4 owner, all you have to do to unlock Bootloader is go to the Google Play Store and download a dedicated Nexus app, BootUnlocker for Nexus Devices.

The Nexus family makes Bootloader unlock much easier / AndroidPIT
BootUnlocker for pre-2014 Nexus Devices
Thanks to this handy application, you will not only be able to unlock your Nexus Bootloader with a single tap, but also have the option to lock it again.
Tutorials of our forum
Our community has prepared great tutorials on how to unlock Bootloader and root Nexus devices. Here are some examples:
LG
Unfortunately, LG complicates things a little, and for their devices, there are no universal methods to unlock Bootloader.
With LG the unloading of Bootloader no to simple / AndroidPIT
If you have an Optimus G, an Optimus G Pro or an LG G2, use the FreeGee app, with which you can also remove LG Security and install Custom Recovery between ClockworkMod and TWRP.
FreeGee Free ** ROOT Required **
If, on the other hand, you have an LG Optimus L5 or L5 Dual, you can use the LG Unlock app:
Also in this case you can install ClockworkMod Custom Recovery.
Tutorials of our forum
Our community has prepared great tutorials on how to unlock bootloader and root LG devices. Here are some examples:
HTC
For HTC devices we can use Kingo's Bootloader Unlock computer program. This software represents the official method for unlocking the brand Bootloader by automatically downloading the unlock code adapted for your device. In addition, we may choose to block it again, all with just one click:

The HTC Unlock Extremely Simple! / AndroidPIT
On the website at the link above, you will find a very easy guide, and no root permissions are required. But if you want to root your HTC anyway, use the program below, also from Kingo:
Sony
For Sony devices you can also use Kingo software, this time specific to Sony:

Sony helps users unlock Bootloader! / AndroidPIT
Also in this case, you can unlock Bootloader without root permissions. It is possible to obtain them in a second moment, with a specific program:
Tutorials of our forum
Our community has prepared great tutorials on how to unlock bootloader and root Sony devices. Here are some examples:
Huawei
With the app Huawei Bootloader UnlockerYou can unlock Bootloader from 7 Huawei devices. The app has been removed from the Play Store but can still be found in alternative stores like Aptoide. To find out if your device is on the list, check out the app description below:

An app unlocks Huawei devices / AndroidPIT
Motorola
Motorola also has an official Bootloader unlocking service on most of its devices. Unfortunately, the instructions and the whole process are still in English:
Tutorials of our forum
Luckily, our community is one of the largest and most participative when it comes to Motorola, and there are several Portuguese tutorials on our forum to help you in the process:
ASUS
ASUS didn't used to make life easier for those who wanted to play with their devices by installing custom ROMs. However, the manufacturer has released a tool that facilitates the unlocking process on Zenfone 2, Zenfone 5 and Zenfone Selfie.
You can download the tool from ASUS official website. In the Driver / Drivers & Tools option, add your device's OS, in this case Android, and click Utilities.

ASUS has released a little help to unlock Bootloader / AndroidPIT
Now that you've unlocked Bootloader, you can flash any ROM on your Android device!
And have you tried to unlock Bootloader from any Android device? Had problems?
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