On telecommunications circuits, disambiguation is a critical function of voice procedure. Due to any number of variables, including radio static, a busy or loud environment, or similarity in the phonetics of different words, a critical piece of information can be misheard or misunderstood; for instance, a pilot being ordered to "eleven" thousand as opposed to "seven" thousand. To reduce ambiguity, critical information may be broken down and read as separate letters and numbers. To avoid error or misunderstanding, pilots will often read back altitudes in the tens of thousands using both separate numbers and the single word (example: given a climb to 10,000 ft, the pilot replies " climbing to One zero, Ten Thousand"). However, this is usually only used to differentiate between 10,000 and 11,000 ft since these are the most common altitude deviations. The runway number read visually as eighteen, when read over a voice circuit as part of an instruction, becomes "one eight". In some cases a spelling alphabet is used (also called a "radio alphabet" or a "phonetic alphabet"). Instead of the letters AB, the words "Alpha Bravo" are used. "Main" Street becomes "Mike Alpha India November street", clearly separating it from Drain Street and Wayne Street. The numbers 5 and 9 are pronounced "fife" and "niner" respectively, since "five" and "nine" can sound the same over the radio. The use of 'niner' in place of 'nine' is due to German-speaking NATO allies for whom the spoken word 'nine' could be confused with the German word 'nein' or 'no'.