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No M1 Mac can boot any OS prior to Big Sur - there simply isn't any binaries for it to run. With intel macs you can boot and install older macOS's up to a cert

No M1 Mac can boot any OS prior to Big Sur - there simply isn't any binaries for it to run. With intel macs you can boot and install older macOS's up to a certain point. Like the previous poster says, it's about what firmware versions you have. For example, if your mac came with Sierra, and through the regular usage and update, you patched it up to Mojave, and installed (or was forced to install by the way, there is no way to reject certain types of updates) firmware updates, you find that all of a sudden you can't re-install Sierra anymore. You'd need to install whatever version your firmware now supports. This behaviour is undocumented and it's case by case - for each mac hardware there is a firmware for it - there are lots of people online who have figured out patch levels are for each revision of hardware, etc - but it's more trial and error to see what older version of macOS you can downgrade to.

Wowfunhappy 6y ago
> For example, if your mac came with Sierra, and through the regular usage and update, you patched it up to Mojave, and installed (or was forced to install by the way, there is no way to reject certain types of updates) firmware updates, you find that all of a sudden you can't re-install Sierra anymore. You'd need to install whatever version your firmware now supports. I have never seen any record of this happening. I'm really quite sure you've always been able to install the earliest OS that your model of Mac shipped with. The odd situation I mentioned above is the closest I've ever seen to a firmware update causing problems (if that's even the cause), and even then the OS runs fine.
- barkingcat 6y ago
  Yah even if it hasn't happened to you yet, be aware that this is something that happens. There actually is a table of firmware versions, etc - once you upgrade your old Mac hardware past a certain firmware, you will no longer be able to run certain versions of macOS "before" the firware was supported. This isn't just between "major" versions, also between "minor" versions of macOS For example, between the current version of macOS Sierra, which is on 10.12.6 (6 being the minor version), if you try to install for example 10.12.x where x is below 6, etc - you might get the infamous ? or the circle with a slash through it on bootup.
  - Wowfunhappy 6y ago
    But I have run 10.12.0 after installing 12.12.6. What models does this happen on?
    - Wowfunhappy 6y ago
      ...Wait, no, I can't say for sure if I'd ever run 10.12.6 specifically. But I ran stock Sierra (10.12.0) after running Mojave, and it's hard to imagine that would result in a more compatible firmware...
      - deleted 6y ago
        [deleted]

Wowfunhappy 6y ago

> For example, if your mac came with Sierra, and through the regular usage and update, you patched it up to Mojave, and installed (or was forced to install by the way, there is no way to reject certain types of updates) firmware updates, you find that all of a sudden you can't re-install Sierra anymore. You'd need to install whatever version your firmware now supports. I have never seen any record of this happening. I'm really quite sure you've always been able to install the earliest OS that your model of Mac shipped with. The odd situation I mentioned above is the closest I've ever seen to a firmware update causing problems (if that's even the cause), and even then the OS runs fine.

barkingcat 6y ago
Yah even if it hasn't happened to you yet, be aware that this is something that happens. There actually is a table of firmware versions, etc - once you upgrade your old Mac hardware past a certain firmware, you will no longer be able to run certain versions of macOS "before" the firware was supported. This isn't just between "major" versions, also between "minor" versions of macOS For example, between the current version of macOS Sierra, which is on 10.12.6 (6 being the minor version), if you try to install for example 10.12.x where x is below 6, etc - you might get the infamous ? or the circle with a slash through it on bootup.
- Wowfunhappy 6y ago
  But I have run 10.12.0 after installing 12.12.6. What models does this happen on?
  - Wowfunhappy 6y ago
    ...Wait, no, I can't say for sure if I'd ever run 10.12.6 specifically. But I ran stock Sierra (10.12.0) after running Mojave, and it's hard to imagine that would result in a more compatible firmware...
    - deleted 6y ago
      [deleted]