"Floating ablaut."

Normal Proto-Indo-European ablaut exhibits three grades: e-grade, o-grade, and zero-grade, whereby a given root (say, *gwhen- "slay") will generally have an e-grade *gwhen-, an o-grade *gwhon- and a zero-grade *gwhn-.

In roots with three consonants, the zero-grade is ambiguous: is *prk'- the zero-grade of *prek'- "ask", or the zero-grade of *perk'- "dig out"?

Schwebeablaut is the phenomenon where sometimes both possible full grades exist for the same root, such as *grebh- and *gerbh- "scratch".