rhotic, there is some variation in its phonetic description. A transcription with the IPA character for an alveolar approximant () is common, though the sound is more postalveolar. Actual retroflexion may occur as well and both occur as variations of the same sound. However, makes a distinction between the vowels of American English (which he calls "rhotacized") and vowels with "retroflexion" such as those that appear in Badaga; , on the other hand, labels both as r-colored and notes that both have a lowered third formant. Because the vowels are articulated with spread lips, spreading is implied for their approximant analogues, . However, these sounds generally have little or no lip-spreading. The fricative letters with a lowering diacritic, , may therefore be justified for a neutral articulation between spread and rounded . In articulation and often diachronically, palatal approximants correspond to front vowels, velar approximants to back vowels, and labialized approximants to rounded vowels. In American English, the rhotic approximant corresponds to the rhotic vowel. This can create alternations (as shown in the above table). In addition to alternations, glides can be inserted to the left or the right of their corresponding vowels when they occur next to a hiatus. For example, in Ukrainian, medial triggers the formation of an inserted that acts as a syllable onset so that when the affix is added to футбол ('football') to make футболіст 'football player', it is pronounced , but маоїст ('Maoist'), with the same affix, is pronounced with a glide. Dutch for many speakers has a similar process that extends to mid vowels: bioscoop → ('cinema') zee + en → ('seas') fluor → ('fluorine') reu + en → ('male dogs') Rwanda → ('Rwanda') Boaz → ('Boaz') Similarly, vowels can be inserted next to their corresponding glide in certain phonetic environments. Sievers' law describes this behaviour for Germanic. Non-high semivowels also occur. In colloquial Nepali speech, a process of glide-formation occurs, where one of two adjacent vowels becomes non-syllabic; the process includes mid vowels so that ('cause to wish') features a non-syllabic mid vowel. Spanish features a similar process and even nonsyllabic can occur so that ahorita ('right away') is pronounced . It is not often clear, however, whether such sequences involve a semivowel (a consonant) or a diphthong (a vowel), and in many cases, it may not be a meaningful distinction. Although many languages have central vowels , which lie between back/velar