# Why should some TeX primitives never be set directly in LaTeX?

In an earlier question More economic and robust command for abstract at the beginning of chapters, David Carlisle told that one should never use directly some TeX primitives. How can I distinguish which primitives are safe to use?

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Well, it depends on what primitive: \hfill is a TeX primitive, but it's surely allowed for use in LaTeX documents. – egreg Apr 5 '14 at 11:57
I assume you refer to tex.stackexchange.com/questions/169719/…? Please focus the question on the specific primitives raised there. – Joseph Wright Apr 5 '14 at 12:03