# Is implicit `\relax` in conditionals explained anywhere in the TeXbook?

I have been relying for years on TeX's behavior where, say `\if1\fi` expands to nothing and `\if1\else\fi` does the same. In the second case TeX inserts an 'implicit' `\relax` when it tries to expand `\else`. I have a 'language-lawyer' type question about it: can this behavior, more or less formally be 'extracted' from the TeXbook? Any reference would be appreciated. Note that I am not interested in the mechanism itself (I found everything I need in 'TeX: the program' a long time ago), just a (more or less) formal reference to it in the TeXbook.

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No, there's no reference to it in the TeXbook. Only in “TeX the program“: look for modules 379, 378, 366 and, especially, 510. You can find the document by doing `texdoc tex` on a TeX Live distribution. As far as I know, the frozen `\relax` is inserted also in `\if1\fi`. – egreg Apr 2 '14 at 21:46
I was afraid of that. As I mentioned above, I am not interested in references other than the TeXbook (I think 'TeX by topic' mentions it, although I am no sure). True about `\if1\fi`, it is just easier to see why TeX has to do it in the second case. – alexsh Apr 2 '14 at 22:00
It's mentioned in section 12.5.3 of TeX by Topic. I rechecked the TeXbook and I can confirm it's not mentioned. – egreg Apr 2 '14 at 22:06

The insertion of the “frozen `\relax`” is not documented in the TeXbook, where `\relax` appears 57 times, mostly in listings. No mention of this insertion is made in the discussion of conditionals.
This is probably the best answer. I have done the search, as well, and, as I mentioned, I have already found the appropriate portion of 'TeX: the program'. I guess I was looking for an indirect way to 'derive' this from the TeXbook. As an example, The TeXbook does not explicitly define what a 'blank token' is but it does introduce the terminology to describe parameter scanning quite precisely. In contrast, I could not find any hint of 'implicit' (or frozen as you call it) `\relax` in it. I will wait a bit longer before I mark this as the answer. – alexsh Apr 3 '14 at 1:27