Shimano vs SRAM
Shimano and SRAM dominate drivetrains, and mixing them is possible but has rules. On cassette and freehub they don't mix: Shimano 12sp uses Microspline and SRAM 12sp uses XD, incompatible geometries. On chain, each brand has its own 12sp profile (Shimano Hyperglide+, SRAM Flat-Top and Eagle) tuned for its own chainrings and cassettes. What historically stops mixing shifters and derailleurs is cable pull (how much cable the shifter moves per click): Shimano and SRAM use different ratios. On 12sp MTB the cog spacing is so similar that the «Mullet» setup exists, but it isn't universal plug-and-play.
«Can I run a SRAM derailleur with Shimano shifters?» is the workshop's eternal question. Short answer: sometimes, and it hinges on a part nobody looks at.
The real friction point
The true limit between brands isn't the cassette, it's cable pull: how much cable the shifter pulls per click. Shimano and SRAM use different ratios, so one brand's shifter won't index the other's derailleur well (bar specific cases). On 12sp MTB the cog spacing is nearly identical, enabling the famous «Mullet» (e.g. one brand's shifter/derailleur with the other's cassette), but each combination must be verified.
What mixes
What you CAN and CAN'T
What you CAN, carefully: on 12sp MTB, thanks to near-identical spacing, running one brand's cassette with the other's chain usually works (keep the chain matched to the cassette). What you CAN'T: put a cassette on the wrong freehub (Microspline vs XD), or expect a Shimano shifter to move a SRAM derailleur given the different cable pull.
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Frequently asked questions
Can I use a Shimano 12sp cassette with a SRAM 12sp derailleur?
On MTB it usually works because spacing is nearly identical (the «Mullet» setup), always with the chain matched to the cassette. Verify your specific combo.
Why won't a Shimano shifter move a SRAM derailleur?
Cable pull: each brand pulls a different amount of cable per click, so the rival's derailleur won't index right.
Does a Shimano chain work on SRAM?
On 12sp each brand has its profile (HG+ vs Flat-Top/Eagle). They work best within their own system; mix them only if you know what you're doing.