To be on cloud nine is to be blissfully happy; an American expression that dates from the mid-20th century. Like other ‘nine’ expressions e.g. dressed to the nines, the whole nine yards, the origin remains obscure. All of them, however, retain some sense of nine as a superlative or implying perfection. The number nine as three times three has long enjoyed mystical connotations. That is why there are nine Muses and why a novena is nine days of prayer etc. In addition, when any number is multiplied by nine, continuing to add up the digits always brings the aggregate of the last two back to nine. This continued fascination with nine remains throughout history. The often-encountered explanation that cloud nine derives from the US Weather Bureau cloud classification, where each type of cloud has nine grades, is regarded as spurious by most etymologists.