Trask (1981): "What was the original source of -ts- and -k-? We cannot be sure, but I would like to suggest that they were prepositions ..."

Atzokoan Trask (1995) mintzo zén gain "-ki" eta "-ts" nola elementu independente zaharrak zein, nonoiz, joanen zirén lotzen ki áditz-partizipio batzuk:

... , it seems highly probable that these two morphs were originally independent elements which eventually came to be incorporated into the participles of certain verbs, ... [Trask, 1995:239-231]

Horretaz ere mintzo ginén herenegun:

..., Trask-en ikuskeran, flexio indirekto horiek akaso etorriko zirén ti forma aske zaharragoak (esan nahi baita: preposizioak), zein aurrena lotuko zirén ki adizki finitoak (jokatuak: "dakar - ki - gu"), eta soilik gerora reinterpretatuko zirén nola aditz-erroak ("ekarki dut"):

One plausible hypothesis is that these morphs appeared first in finite forms, perhaps by the incorporation of earlier free forms, ... [Trask, 1995:230]

Gaur gogoratu nahi dugu Trask-en referentzia zaharrago bat ("Basque verbal morphology", 1981): 

What was the original source of -ts- and -k-? We cannot be sure, but I would like to suggest that they were prepositions ... [Trask, 1981:289]

non Trask mintzo da gain "-ts-" eta "-k"- soilak.