Dual Booting

This page could use some expansion. If you'd like to help out, feel free to edit this article!
In particular: This huge list could be split up into more specific tutorials. Perhaps include the contents from Use PostmarketOS Community pre-built image on your device here?

for a better guide, see Dual Booting/WiP

Dual Booting, also sometimes called Multiboot, is a process by which you can install multiple operating systems on your device, and select between them when booting it up.

postmarketOS currently might able to dual booting through recovery (kind of)

There are a few ways this can be achieved:

It is might be possible to use Safestrap recovery mode 's rom solt feature to dual boot , currently Motorola Droid 4 (motorola-maserati) is only device that utilize Safestrap to boot
On the Nokia N900 and many device that use uboot (like MangoPi MQ-Pro (mangopi-mq-pro) , Xunlong Orange Pi 3 (xunlong-orangepi3)) , u-boot provides a mechanism to select the OS. See Nokia N900#Installation and See bootmenu on uboot doc
One more way with u-boot: U-Boot_porting#Implement_multiboot_involving_Android_in_a_device_aport
On some amlogic device 's uboot . there are a multiboot mode : Amlogic#Boot process
On fastboot devices that have an SD card, it is possible to install postmarketOS on the SD card, and boot through pmbootstrap flasher boot. This has the advantage of not modifying anything on the phone itself, but a USB cable and a host device (like a computer) are needed to boot the device with this procedure. During boot the initramfs script will automatically search the list of available partitions for the boot and root partitions either by name or by their partition identifier and continue booting when found.
On most devices, the recovery partition holds a recovery image (or fastboot boot image (on some deivce)) . With the postmarketOS boot image flashed to the recovery partition and the root image flashed to either an internal partition or an sdcard, you will able to dualboot but doing that will make you lose the android recovery mode (this works on xiaomi-santoni and samsung-degaswifi)
On devices with lk2nd the boot image is loaded from an ext2 boot partition either on EMMC or SD card. Since lk2nd keeps backwards compatibility with android boot images, you can flash your android boot.img via lk2nd and it will boot that image unless an SD card with postmarketOS is inserted.
Some uefi Implementation for android phone like (efidroid,Renegade Project) can be used together with postmarketOS (see Guides for Dual boot pm os with other uefi based os via Renegade Project (with included guides for booting pm os via Renegade Project) on oneplus 6/6t but some part of it can be reused on some other device as well.)
On uefi / bios based device , grub can be used for dual booting (see Dual boot with Windows on arch wiki)
On newer android phone Android AB Slots can be used for dual boot.
Probably multiboot can be implemented with any second stage bootloader, capable of running linux (for example lk2nd,grub)
you can also use stowaways rootfs by putting all files in .stowaways/pmos and forcing that partition to be used as root

See also