Glottalic theory
The glottalic theory is that Proto-Indo-European had ejective or otherwise non-pulmonic stops, *pʼ *tʼ *kʼ, instead of the plain voiced ones, *b *d *ɡ as hypothesized by the usual Proto-Indo-European phonological reconstructions. A forerunner of the theory was proposed by the Danish linguist Holger Pedersen in 1951, but he did not involve glottalized sounds.