The glottalic theory says that Proto-Indo-European had ejective stops instead of voiced ones, namely p’ t’ k’ rather than b d g, to the traditional reconstruction of the Proto-Indo-European sound system.
The glottalic theory was made in the United States by Paul Hopper and in the Soviet Union by Thomas V. Gamkrelidze and Vyacheslav V. Ivanov. In its earliest theory, made by the Danish linguist Holger Pedersen, it did not involve the use of glottalized sounds. While earlier linguists, such as André Martinet and Morris Swadesh, had seen the potential of substituting glottalic sounds for the supposed plain voiced stops of Proto-Indo-European, the request stayed speculative until more evidence for it was published in 1973 by Hopper in the journal Glossa and by Gamkrelidze and Ivanov in the journal Phonetica.
- Paul J. Hopper, "Glottalized and murmured occlusives in Indo-European." Glossa 7:2:1973, 141-166.
- Thomas V. Gamkrelidze and Vjacheslav V. Ivanov, Indo-European and the Indo-Europeans, translated by Johanna Nichols, 2 volumes. Berlin and New York: Mouton de Gruyter, 1995.
- Robert S.P. Beekes, Comparative Indo-European Linguistics. John Benjamins, 1995.
- Anthony Fox, Linguistic Reconstruction. Oxford, 1995.