in Hittite; they are used somewhat interchangeably in some words, while other words are spelled consistently. The contrast in these cases is not entirely clear, and several interpretations of the underlying phonology have been proposed.

Similarly, the purpose of inserting an additional vowel between syllabograms (often referred to as "plene writing" of vowels) is not clear. Examples of this practice include the -a- in is-ha-a-as "master" or in la-a-man "name", ú-i-da-a-ar "waters". In some cases, it may indicate an inherited long vowel