What is a 1-4-5 Progression?

The 1-4-5 or, more correctly, the I-IV-V chord progression is the most common chord progression in all of pop music.

The I-IV-V is what classical composers call "figured bass", which is also the fundamental structure of blues, and by extension rock, and is the basis for the "Nashville Notation System" that is very common in country music. This is a way of denoting chords that can easily be adapted to any key without having to transpose. The idea is based around "scale degrees". The western scales all have seven degrees which can be represented by a number from I to VII. Thus, a scale can be written as I II III IV V VI VII I. in the key of C, the scale would be C D E F G A B C. In Bb, it would be Bb C D Eb F G A Bb.

The progression of I-IV-V states that you first play a I chord, followed by a IV chord and finally a V chord. In the key of C, this would translate as C-F-G. In Bb, it would be Bb-Eb-F. In A, it's A-D-E.

The sonority of a chord can also be notated using figured bass. At its most basic level, the capital roman numeral represents a major chord, while a lower-case roman numeral would represent a minor chord, thus I-ii-V-IV-V7 would be, in the key of C, C major-D minor-G major-F major-G major seven.

As stated before, this system makes it easy to play any chord progression in any key without having to transpose. For instance, if a guitar player knows a particular song in the key of F, but plays a gig with a singer who is uncomfortable singing in that key will have to transpose. If the singer is more comfortable singing in Bb, it's much easier to say "play I- IV-iii-V7 in the key of Bb", then to say "play F maj-Bb maj-A min- C maj seven, but transpose it into the key of Bb" which, for the record, would be Bb maj-Eb maj-D min-F maj seven.