Comment by mort96

The relevant Wikipedia page (https://en.wikipedia.org/wiki/36-bit_computing)indicates that 6x6 was the most common, but that 5x7 was sometimes used as well.

... However I'm not sure how much I trust it. It says that 5x7 was "the usual PDP-6/10 convention" and was called "five-seven ASCII", but I can't find the phrase "five-seven ASCII" anywhere on Google except for posts quoting that Wikipedia page. It cites two references, neither of which contain the phrase "five-seven ascii".

Though one of the references (RFC 114, for FTP) corroborates that PDP-10 could use 5x7:

    [...] For example, if a
    PDP-10 receives data types A, A1, AE, or A7, it can store the
    ASCII characters five to a word (DEC-packed ASCII).  If the
    datatype is A8 or A9, it would store the characters four to a
    word.  Sixbit characters would be stored six to a word.

To me, it seems like 5x7 was one of multiple conventions you could store character data in a PDP-10 (and probably other 36-bit machines), and Wikipedia hallucinated that the name for this convention is "five-seven ASCII". (For niche topics like this, I sometimes see authors just stating their own personal terminology for things as a fact; be sure to check sources!).