The phonetic values of the laryngeal phonemes are disputable; various suggestions for their exact phonetic value have been made, ranging from cautious claims that all that can be said with certainty is that represented a fricative pronounced far back in the mouth, and that exhibited lip-rounding up to more definite proposals; e.g. Meier-Brügger writes that realizations of = , = and = or "are in all probability accurate". Another commonly cited speculation for is (e.g. Beekes). Simon (2013) has argued that the Hieroglyphic Luwian sign *19 stood for (distinct from ) and represented the reflex of . It is possible, however, that all three laryngeals ultimately fell together as a glottal stop in some languages. Evidence for this development in Balto-Slavic comes from the eventual development of post-vocalic laryngeals into a register distinction commonly described as "acute" (vs. "circumflex" register on long vocalics not originally closed by a laryngeal) and marked in some fashion on all long syllables, whether stressed or not; furthermore, in some circumstances original acute register is reflected by a "broken tone" (i.e. glottalized vowel) in modern Latvian.