this is an ex-blog
Numbers ending in 15 or 16 would be written as yud-he (10+5) and yud-vav (10+6), but the letters "yud he vav he" spell out the name of God and for religious reasons are not used. Instead, by convention, tet-vav (9+6 טו) and tet-zayin (9+7 טז) are always used.
Well ok, there is another exception- Some numbers spell out a word with strongly negative or positive connotations. In these cases, the order of the letters might be changed. The number 18 for example, yud-het, uses the same letters as the word for life het-yud. So instead of יח, you may see חי.