WHAT IF I CAN’T RUN IT?

Apple has a well-earned reputation for making computers that last a long time, largely because it doesn’t often change the system requirements when moving from one operating system to the next. With Sierra, though, it has raised the minimum requirements for the first time since 2014’s Yosemite (10.10).

To run Sierra, which is free from the Mac App Store, you need at least 2GB of memory and 8.8GB of available storage. You need a MacBook or iMac from late 2009 or later; a Mac Pro, Mac mini or MacBook Pro from mid-2010 or later; a MacBook Air from late 2010 or later; or a 12-inch MacBook.

If your Mac is too old, you won’t be able to install Sierra — not officially, anyway; there are third-party tools to force the installer to run, but consider that trustworthiness and that stability isn’t guaranteed. Still, there’ll be security updates for El Capitan for a while yet, so make sure you keep it up to date.