The main reason why a prefix name would differ has to do with pronunciation and vowels: If the first letter of the unit name is a vowel and the pronunciation is difficult, the last letter of the prefix is omitted. For example, a metric measurement of 100 ares (2.471 acres) is a hectare (not hectoare) and 1 million ohms is a megohm (not megaohm). There are exceptions, though, especially if the resulting prefix and unit sound fine, such as a milliampere. There are even times that another letter is added to make it easier to roll off the tongue. For example, an “l” is added to the term for 1 million ergs, making it a megalerg, not a megaerg or megerg.