The form *structum* seems to have "c" by analogy to stems that had labiovelar consonants in Proto-Indo-European, as Alex B. says. > In *frūctus* (from *fruor*) the Indo-European present stem ended in a labio-velar *gʷ,* but various analogies no doubt account for *strūctus, flūxus,* old *flūctus, uīctum* (from *struo, fluo, uīuo*) (*Vox Latina,* W. Sidney Allen, Second Edition, p. 69) (First published 1965, Second edition 1978) There is also a relevant footnote on this page: > 2. Old Latin [for *fīgo*] is in fact *fīuo,* with *u* from Indo-European *gʷ.* But note *nīxus* (and *nictare*) from *(co)nīveo,* where *u* is from Indo-European *gʷh.* In *[Latin and Greek Accidence][1],* by John Perkins, I found the following examples of verbs that have a velar consonant only in non-present stems: >- fluo, fluxi, fluctum/fluxum - struo, struxi, structum - vivo, vixi, victum (p 36) >- fruor, fructus/fruitus - nitor, nisus/nixus (p 37) "Latin Verbs in -uo, -uere", by Oswald Szemerényi, says > Two [*of the verbs ending in -uo, -uere*] show in the perfect and the PPP a velar stem: >> *fluo -ere fluxī fluctum* (later *fluxus*) >> <br>*struo -ere struxī structum.* > No doubt ... the same type is seen in >> *fruor fruī frūctus* (p. 11) Wiktionary (accessed 24 June 2017) gives the following etymologies: - [fluo][2]: "From Proto-Italic *flowō" (cites De Vaan), "from Proto-Indo-European *\*bʰlewgʷ-,* from *\*bʰlew-*" - [struo][3]: "From Proto-Italic *strowō" (cites De Vaan) "with spurious *c* in *struxī* and *structum*" - [vivo][4]: "From Proto-Italic *gʷīwō [...] The *x* and *c* in *vīxī* and *vīctum* were introduced by analogy with other verbs." I left out what Wiktionary says about the Indo-European etymology because my understanding is that this is a somewhat troublesome root to analyze - [fruor][5]: "From the Proto-Indo-European *\*bʰruHg-*" (De Vaan is listed in the "references" section, but there's no explicit citation and no Proto-Italic form given) - [nitor][6]: "From Proto-Indo-European *\*kneygʷʰ-* (“to bend, to droop”). Cognate with *nicō, connīveō, nictō*" As you might have noticed, Wiktionary, unlike Allen, doesn't write a macron on *structum,* and there is a note in the Wiktionary entry for *struo* saying: > Please note that there is a disagreement over whether or not there is a macron on the third and fourth principal parts and the subsequent verb forms from these (*strūxī* for *struxī* and *strūctum* for *structum*). However, there is no citation for this statement, so I don't know what the basis for the disagreement would be. *[The Latin language; a historical outline of its sounds, inflections, and syntax][7],* by Charles E. Bennett (1907) says > Gellius [...] testifies (*Noctes Atticae,* ix. 6) to the quantity of the vowels of *āctus, lēctus, ūnctus,* and in xii. 3. 4 to that of ***strūctus*** (p. 52) and > the Romance languages [...] point to *cīnxī, distīnxī, exstīnxī, fīnxī, pīnxī, **strūxī,** tīnxī, ūnxī* (p. 53) but perhaps there are some issues with this evidence, or opposing pieces of evidence. Bennet doesn't indicate any inscriptional evidence (e.g. use of the [apex][8]) for vowel length in this word. ###Further reading Some links to further literature that seems relevant that I found by Googling, but that I haven't processed yet: - "[Latin Verbs in -uo, -uere][9]", Oswald Szemerényi, in *Italic and Romance Linguistic Studies in Honor of Ernst Pulgram* (1980). This seems pretty detailed! I can only see part of it through Google Books, but it looks worth a read. - "[On some Slavic words in *-č*: *k* from laryngeal?][11]", by Jens Elmegård Rasmussen (1988) - "[Lachmann's Law (Part 2)][12]", by Roman Sukač (2013) - "[On Latin *strāgulum* and *strāges*: *-g-* and analogy][10]", by Kanehiro Nishimura, in *Words and Dictionaries: A Festschrift for Professor Stanisław Stachowski on the Occassion of his 85<sup>th</sup> Birthday* (2015) [1]: https://books.google.com/books?id=KwZyJh3QgDEC&pg=PA36&lpg=PA36&dq=fluo%20fluxum%20fluctum%20fruor%20fructus&source=bl&ots=ZXCASg4GJQ&sig=fMotdFecey-Rwu5FVMOYhRmFwyQ&hl=en&sa=X&ved=0ahUKEwjjyZud5dfUAhUL5oMKHSpYCBsQ6AEILjAB#v=onepage&q=fructus&f=false [2]: https://en.wiktionary.org/wiki/fluo#Latin [3]: https://en.wiktionary.org/wiki/struo#Latin [4]: https://en.wiktionary.org/wiki/vivo#Latin [5]: https://en.wiktionary.org/wiki/fruor#Latin [6]: https://en.wiktionary.org/wiki/nitor#Latin [7]: https://archive.org/stream/latinlanguagehis00benn#page/52/mode/2up [8]: https://en.wikipedia.org/wiki/Apex_(diacritic) [9]: https://books.google.com/books?id=1mVwAuuC8GoC&pg=PA12&lpg=PA12&dq=vowel%20length%20%22struxi%22&source=bl&ots=-LmXUZCyDT&sig=2uwMYrKtuV_XXFA-xrqOdclpYCA&hl=en&sa=X&ved=0ahUKEwjLwrn6odjUAhVExoMKHZhxDbkQ6AEINDAF#v=snippet&q=struo&f=false [10]: https://books.google.com/books?id=SoQIDgAAQBAJ&pg=PA234&lpg=PA234&dq=vowel%20length%20%22struxi%22&source=bl&ots=XVqslgIwak&sig=ufCsDbYWCXFelNLOLVQfkdFcGt4&hl=en&sa=X&ved=0ahUKEwjLwrn6odjUAhVExoMKHZhxDbkQ6AEIJjAA#v=onepage&q=vowel%20length%20%22struxi%22&f=false [11]: https://books.google.com/books?id=--ssbDUSJfIC&pg=PA344&lpg=PA344&dq=vowel%20length%20%22struxi%22&source=bl&ots=OnRD8wQlyO&sig=LXJvc7ZmMZalhjEHkOk5TfjlpD8&hl=en&sa=X&ved=0ahUKEwjLwrn6odjUAhVExoMKHZhxDbkQ6AEIMTAE#v=onepage&q=vowel%20length%20%22struxi%22&f=false [12]: https://digilib.phil.muni.cz/bitstream/handle/11222.digilib/127146/1_LinguisticaBrunensia_14-2013-1_2.pdf?sequence=1