For ordinary trademarks, distance between industries is a real defence. The same word can be an airline and a tap manufacturer because nobody buying a tap wonders about the airline. See the different-industry question.
Famous marks are the exception, and the exception is large.
Dilution, and why it changes the answer
Federal law gives the owner of a famous mark a claim that does not depend on likelihood of confusion at all. It does not matter that your customers know perfectly well you are not them. The theory is that the mark itself is an asset, and that its power comes from pointing to exactly one source. Two kinds of harm are recognised.
Blurring is the erosion. If the word starts appearing on unrelated goods from unrelated sellers, it gradually stops being a unique identifier, whether or not anyone was ever confused.
Tarnishment is association with something that reflects badly on the mark.
The threshold is high. The mark has to be famous to the general consuming public of the United States, not merely well known in its own industry. Being the best-known brand among commercial roofers is not fame for this purpose. A handful of brands clear the bar easily and a great many that feel famous do not.
The practical consequences
- Distance stops helping. The further your goods sit from theirs, the better your confusion argument and the more irrelevant it becomes.
- Evidence of no confusion stops helping. Dilution does not ask.
- Enforcement is systematic. Owners of famous marks watch the register continuously and oppose as a matter of routine, because a mark famous enough to be diluted is a mark that must be policed to stay that way.
Parody and homage
Parody can be a defence and it is narrower than most people assume. The successful version comments on the original and is plainly understood as not coming from it. The unsuccessful version, which is far more common, borrows a famous mark to sell an unrelated product and calls the borrowing a joke. Courts distinguish between commenting on a brand and using a brand, and that distinction does most of the work.
Homage is not a legal category at all.
What to do if your name is close by accident
Assume it will be found. Register monitoring at that level is automated and thorough, so the question is not whether anyone notices but what happens when they do.
Two things genuinely help. Move further away at the naming stage, which costs nothing now and a rebrand later. And document independent creation, because the story of how you arrived at the name is worth having written down at the time rather than reconstructed under pressure.
If a famous mark sits anywhere near your shortlist, this is the point at which a conversation with a trademark attorney is cheap relative to what it protects.