Today, the reconstruction of consonantal sounds to explain what was reconstructed before as uncertain vocalic schwa indogermanicum or schwa primum is firmly accepted in Indo-European (IE) studies in general, and there is a general agreement on where laryngeals should be reconstructed (Keiler 1970).
Even the number and quality of those laryngeals is today a field of common agreement, although alternative number of laryngeals and proposals for their actual phonemic value do actually exist. Reconstructed laryngeals are valid only for the oldest reconstructible stage using comparative grammar, i.e. Middle Proto-Indo-European or Proto-Indo-Anatolian (Kloekhorst 2016; Schmidt 2011; Jasanoff 2003)[xxiii], and potentially also Indo-Uralic (Hyllested 2009; Kloekhorst 2008).
These laryngeals are in most cases notated as *h1, *h2, *h3 but sometimes also with their assumed realisation *he, *ha, *ho, or phonetic inventory, *ʔ/*h, *χ/*ħ, *ɣw/ʕw. A more traditional representation is found in *a1, *a2, *a3, or *ə1, *ə2, *ə3. Sometimes, a vocalic quality is assumed, *Ee, *Ae, *Oe.
Their evolution during Late Proto-Indo-European (LPIE), after the separation of Anatolian, is often assumed as a loss or deletion with certain common outputs in the daughter branches or proto-languages (Adrados 1998; Bomhard 2018; Koch 2013). However, it has also been stated that the three laryngeals might have survived until the final phase of LPIE (Rasmussen 1999). A certain support is found for the survival of laryngeals until after the separation (Cogwill 1960), but the general view is that they disappeared completely, leaving only indirect traces in historical languages (Sanker 2015).
As Clackson (2007) sums up: “Particularly puzzling is the paradox that laryngeals are lost nearly everywhere, in ways that are strikingly similar, yet apparently unique to each language branch. We can of course assume some common developments already within PIE, such as the effect of the laryngeals *h2 and *h3 to change a neighbouring *e to *a or *o, but the actual loss of laryngeals must be assumed to have taken place separately after the break-up of the parent language (…) it would have seemed a plausible assumption that the retention of *h2, and possibly also *h1 and *h3, is an archaism of Anatolian, and the loss of the laryngeals was made in common by the other languages.”
Chronologically, there is no commonly agreed scheme as to the maintenance of laryngeals in daughter languages. Whereas there is some common ground whereby laryngeals were lost by the time when Late Indo-European languages were written down (Rasmussen 1999; Sukač 2014), its survival has been supported for certain late proto-languages, e.g. for Slavic as late as Charlemagne’s times (Kortlandt 1975).