Take for example the melody C D E F G A B in the key of c major. If i play the melody in thirds, I have been told that you would play it C E; D F; E G; etc. but some of those form minor 3rds and some of those form major thirds. How do you know whether to play a minor or major 3rd to stay in the scale ?
Generally one stays in the scale which entails using some major and some minor thirds. There are a few exceptions. When approaching a cadence (usually at the end of phrases) one may modify a minor third to a major third to produce a secondary dominant. Of course this procedure assumes that a secondary dominant is appropriate at that point (and that the other musicians, if any, agree with your choice). If you are improvising, you can obviously make your own harmonic changes. Mostly, you play the lower third that fits the current scale. The object of the thirds should be to support the melody not change it.
It depends what you mean by "trick".
Start by remembering the order of thirds when playing any major scale, which is:
Major Minor Minor Major Major Minor Minor
That's not very hard to remember. It's 1 major, 2 minors, 2 majors, 2 minors.
Just play that up and down for a while until you've got it memorized.
Then try jumping around a bit, e.g. Cmaj Emin Gmaj back to Cmaj (I'm talking about playing thirds here, not full chords). Pretty soon your visual, audio and touch senses will work together to remember the patterns.
Finally, I would recommend trying to improvise on top of a song (you can use a YouTube backing track or a patient friend). Preferably start with a song that doesn't have any chords changes (old blues-based songs are best for this - try Aretha Franklin "Chain of Fools"). Play super simple melodies (e.g. three or four notes), harmonizing the melody using thirds. You may be surprised how your ear helps guide you which notes to play.
I look to the scale to determine which notes become roots for Major Chords and which become roots for minor or diminished chords. The I, IV, and V, become Major chords based on the fact that the root to third interval is a major third. The second, third, sixth, and seventh notes build minor chords because the root to third interval utilizes a minor third interval in order to utilize only scale notes in their structure. The seventh note builds minor7flat5 chord which also utilizes a minor third interval. That means you can determine the 1st, 4th, and 5th notes of the scale, play Major thirds against those notes and play minor thirds against all the others. This is to be looked at as a guideline and not a hard rule, but it might qualify as a shortcut or trick. With practice it becomes intuitive.
Here's another take on the "Trick to figure out what the proper note to play is when playing in thirds", which is an extension of David Bowling's comment:
practice. Ultimately, the trick to grokking anything in music is study & practice.
To stay in the scale, play notes that are in the scale! The 'trick' is to know what notes you're playing, rather than thinking of thirds as isolated objects.
There are two main ways. One of them is what has been mentioned; knowing your scale degrees and determined the value from that. The other way is what I prefer; simply memorizing your intervals. At first glance it seems a lot to memorize 12 major thirds and 12 minor thirds, both in flats and sharps, but if you use chunking it becomes easier. On the first line are the major thirds, of which three notes combine to create an augmented chord. The cool thing about augmented chords is that they're symmetrical, so if you invert it the note is still a major third from the last note. 4 chords times 3 notes equals 12 notes. Same thing on line two; the minor thirds stack to make 3 fully diminished 7th chords, which are also symmetrical. 3x4=12.
Memorizing it in chunks like this should help in memorizing the individual intervals and how they all relate to each other.
The ease depends a bit on the instrument that you are using.
You mention piano in your tags and this is one of the easier ones. As others have said, just stay in the same key / scale. If that scale is C major then it is very easy, just stick to the white notes. If it is some other major key then it is slightly more tricky: sometimes one finger will be on a white key and one on a black, both on white, or both on black but, all the time, both will be on a note from the scale. So, if you are using D major then any F or C should be sharp whether it is the upper or lower note.
On the guitar, it can be a bit harder, you will need to understand the scales rather better. Playing with a fixed interval of a major third or a minor third would be much easier but it will sound very different and, to many ears, much less nice. Conversely, deliberately doing this on the piano would actually be harder.
I don't play the harp but I expect that it would come quite naturally and anything else would be hard.
Obviously, you cannot play two notes at once on an instrument such as a flute but two instruments could. The odd thing here is that if the lower part is played alone then it will sound a bit odd, not just the tune transposed down a third.
Minor keys add some complications since the 6th and 7th notes will be sharpened in some contexts. Some knowledge of the theory of music or just some experimentation will be required.
It is pretty easy. The "trick" is the following: you ought to know in which key the song you are playing is. From there it is easy. Let us take C Major for example, it goes like this:
- C D E F G A B
If you look either to the right (upper third) or to the left (lower third) from the base tone, thirds are always, well, the third note. If you look to the right, for C that would be E, for D that would be F, etc. Likewise, if you look to the left, for C that would be A, for D that would be B, etc.
That means, for upper thirds you can write something like this:
C D E F G A B <- base notes E F G A B C D <- corresponding thirds
What I did here is, I just rotated C Major scale three places to the left to get corresponding thirds.
Likewise, you can do a similar thing to get lower thirds, just rotate a scale three places to the right:
C D E F G A B <- base notes A B C D E F G <- corresponding thirds
You can notice here that when we are talking about upper thirds, C to E is major third, but when we are talking about lower thirds, A to C is minor third. That should not bother you for now.
Now, if your song is in C Major and its notes are:
- C C E E A A G ...
you can play corresponding upper thirds:
- E E G G C C B ...
or corresponding lower thirds:
- A A C C F F E ...
and both ought to sound good. That's it in its most basic form. You can do this for any scale, you just have to know in which scale your song is, and of course which tones are being played in a melody.
Now, for some simple guidelines:
- basically, you do not mix upper and lower thirds
- as the other answer said: as you approach cadence you could toss in some other tone to create a tension, but I think you should experiment with that later when you have more experience
- when you are resolving a piece (you are at the end of it), and pieces usually finish with the same tone as the key they are in, it is sometimes nicer to play that tone instead of third
Of course, these are just guidelines, you ought to experiment yourself and see what fits, but for starters this will be enough I think.
You have to know your scales absolutely by heart to know how to play it in thirds. You don't have to think about whether they are major or minor thirds, you just have to know what notes belong to the scale. It would be helpful to practice your scale in alternating thirds. For example, for C Major, practice going up by thirds from each scale degree. C-E,D-F,E-G,F-A,G-B,A-C,B-D,C. Then back down, C-A,B-G,A-F,G-E,F-D,E-C,D-B,C. Things like this technique will help you to really learn the associations between the notes. The same can be done for all intervals. Additionally, you can play the whole scale in parallel thirds. Do this for all your scales. Diligent practice is the only way to be able to do this quickly.