scales

[henrikhank]

If a song is in F-Major is it also in D-minor?

[j_menz]

No. The tonic of Dm is D, the tonic of F major is F.  The chords and scales in each key relate to the tonic, so the relationships are different in the different keys.

[jamesplayspiano]

What j_menz said

Also, just a side note: if you're not sure which key you're in, between a major or its relative minor key, there are two common hints that are USUALLY pretty dependable. The biggest thing is that most (traditional/tonal) pieces end on the "I" chord, and very often with the melody on the tonic. That means that, very often, the bass note AND the melody note are the same as whatever key you're in.

Second, in minor keys it is very common to raise the leading tone, which comes out as an accidental to the key signature. From that minor key's perspective, it's a raised seventh scale degree, but from the parallel major's perspective it's like a raised fifth scale degree. Example: in your F major vs. D minor example, look for accidentals. If you see lots of C#s, there's a good chance you're actually in D minor. That C# is the raised seventh note of the scale, from D's point of view, or a raised fifth, from F's point of view.

[brianvds]

And quite apart from all the theory, of which I understand very little, a piece in a minor key usually SOUNDS very different from a piece in a major key. If it has an F major key signature but sounds kind of minor, then it is likely to be in D minor.

[keyofc]

henrick,
They both share the same notes -
d minor starts on the the 6th of the scale of F.

[keypeg]

A piece that is in F major often modulates to D minor or other related keys.  The piece itself will be in D minor, usually starting and ending in that key and on the tonic note and its chord.  But along the way it can visit other keys, which you'll see through the accidentals and hear, because suddenly there seems to be another main note that it wants to come home to.

[49410enrique]

also look for your accidentals, sometimes pieces can use a V of V so in d minor, the V chord is A major and the V of A major is E major, an E major  hord resolving to A major chord is almost a dead giveaway you are in D minor territor, would be very strange animal in F unless you're in 20th century territory.

likewise, the V in F major is C major, and V of C major is G major, so  in F you would be more likely to encounter a G major chord going to C major is more indicative of F major, as in D minor the you would have a minor iv not a major IV.

the example above could also include 7ths.

also is your vii to I or i fully or half diminished and is your 6th tone a Le vs La. taking these into account can help you know where you are or headed or coming from tonally.

note. you have to look at the context, sometimes you can get a V of ____ just about any chord, also vii of a chord in strange places, ie Beethoven was really good about sometimes surprising you with unexpectied chords, even for just a beat or two, not quite a full modulation.