Perfect 4th, A4/d5 and Perfect 5th

P4

The Perfect 4th is five half steps apart. It is most
easily recongnized as the first two notes in "Here Comes
the Bride". It was considered a sacred interval in medeval
times which earned its title of "perfect". The perfect
fourth is also well known as the "Amen" at the end of
many hymns which are still sung in churches today. It has
a "major" sound but also has an open quality to it and
some have even described it as meditative.

A4/d5 Tritone

The Augmented 4th or diminished 5th (notice the
capitol A and lower case d) are both six half steps apart.
The Augmented 4th is a Perfect 4th which is raised or
augmented. The diminished 5th is a Perfect 5th which is
lowered or diminished. They are in fact the same note.
When two notes have the same played pitch they are
called enharmonic.
This interval is easily recognized as the first two notes in
"The Simpsons" theme song. The tritone has a dissonant
or turbulent quality and was called "the devil in music"
due to this "disturbing" quality.

P5

The Perfect 5th is 7 half steps apart.
It is recognized as the first two notes of the Flintstones'
theme song. Like the perfect fourth this has an open and
meditative or zen like quality to it.