Roman numeral code: V / V
Scale notes: 2, #4, 6
Attributes: boldness, power, surprise, unexpected
5 of 5, also called the secondary dominant, and abbreviated as 5/5, adds power and excitement to music, because it is unexpected and ventures outside the normal bounds of the home, or tonic, key. It is frequently heard in all types of music.
5/5 is a chord borrowed from a different key: the key defined by the 5 chord. It is the 5 harmony of that key. So 5/5 bears the same relationship to 5 that 5 bears to 1.
5/5 contains a note (#4) that is not part of the tonic key scale. Notice how the chord sounds like it is outside the confines of the original key, and serves to accent the 5 harmony.
The sequence 5/5 -> 5 -> 1 is an especially powerful cadence.