HG35mm and HG36.8mm freehub bodies are the two most common and most confusing freehub body "standards" because of the wide range of nicknames they're called.
Because of this confusion we choose to call them HG35mm and HG36.8mm. HG means Hyperglide, a Shimano term from 1990, and then the overall length of the freehub body . This means it's easy to measure your own cassette or freehub body to verify what you've got or what you need.
HG35mm is commonly known as "Shimano 10 speed" or "Shimano 11 speed mountain" but it also fits all Shimano-branded cassettes going back to the 7 speed days, and extending into 11 speed Shimano mountain bike and some (but not all) 11 speed Shimano road bike cassettes.
Many Sram cassettes also use the HG35mm freehub body "standard" including most 10 and 11 speed Sram road cassettes, and NX model Sram mountain bike cassettes.
HG36.8mm was introduced in 2012 when Shimano introduced the 11 speed DuraAce groupset. The spline pattern is the same as HG35mm but, you guessed it, 1.8mm longer. Most (but not all) 11 speed Shimano road and gravel cassettes require an HG36.8mm freehub body. All Shimano 12 speed road (but not mountain bike) cassettes also need an HG36.8mm freehub body.
See, we told you it was confusing 🙂
If you're not 100% sure what freehub body you need we suggest you carefully step through our "what freehub body do I need" tool to find out.
HG35mm vs HG36.8mm Freehub Bodies
An HG36.8mm freehub body is identical to HG35mm other than having and additional 1.8mm on the splines. This means that an HG35mm cassette will fit onto an HG36.8mm freehub body when a 1.8mm spacer is fitted behind the cassette.
Please note the opposite is not true: There is no way of fitting an HG36.8mm cassette onto an HG35mm freehub body.