HG vs XD vs Microspline
The three modern freehubs don't interchange, and picking wrong is the #1 buying mistake when changing cassettes. HG (Shimano, 9 splines) takes 8-11sp cassettes with an 11T minimum cog: universal and cheap. Microspline (Shimano, 23 splines) is for its 12-speed MTB cassettes with a 10T cog. XD/XDR (SRAM, threaded) is for SRAM cassettes with a 10T cog (XD on MTB, XDR on road). Simple rule: a 10-tooth cog forces Microspline (if Shimano) or XD (if SRAM); an 11T fits HG.
Before buying a new cassette, this is the only question that matters: which freehub does your wheel have? Solve it here in 30 seconds.
The right question
The question isn't «how many speeds?», but «which brand and what minimum cog?». If your smallest cog is 10 teeth and it's Shimano, you need Microspline; if SRAM, XD (or XDR on road). If the minimum is 11T, it's almost certainly HG. Always confirm your wheel's freehub before buying the cassette: changing it is sometimes just swapping the freehub body.
All three, at a glance
What mixes and what doesn't
HG and Microspline are both Shimano but do NOT interchange. Microspline (Shimano) and XD (SRAM) are both 12-speed MTB but use different geometries. The only «easy» migration is replacing the freehub body if your hub allows it (DT Swiss, Hope, Mavic…).
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Frequently asked questions
How do I know which freehub I need?
Check the cassette brand and its smallest cog. 10T Shimano → Microspline. 10T SRAM → XD (MTB) or XDR (road). 11T → HG.
Can I move from HG to Microspline or XD?
Yes, if your hub lets you change the freehub body. Many quality hubs sell it separately; sealed generic hubs don't.
Which is best?
None is «best»: they depend on your groupset. HG is cheapest and universal; Microspline and XD exist to allow modern systems' 10T cog.