Which factors contribute to how well we retain our repertoire

[michaeljames]

Having played for so many years...decades...I find something very curious:  Why do some studied pieces last for a lifetime, while others disappear? 

I was going through some of the recordings on PianoStreet and was reminded of pieces I've studied through the years.  Feeling nostalgic, I sat down at the piano to play several of them.  While some were still "there," others were like I'd never seen them before!

So, to my learned brethren, how many of you are afflicted by this phenomenon?  And how many of you are not afflicted?

Thanks. 
Michael in Minnesota

[pianoman8]

Everyone loses a piece to a degree after they haven't played in a while. For me, it depends on how much I practiced the piece in the past, and how long I worked on for.

[j_menz]

I find it is a matter of the composer as much as the piece. Some of them seem to sink in and become part of one, even if not played for years, and others  seem to disappear almost on playing.  Bach and Beethoven for me are forever, with Chopin, on the other hand, it's like a new piece every time I open it.

[faulty_damper]

It's called forgetting and everyone forgets as time drags on.  The pieces that tend to withstand time are the ones I play in my mind most often.  But once that stops, it, too, will drop out of my memory.

[liszt1022]

The ones I remember best are the ones whose theory and chord structure I understand. Understanding how tendency tones resolve and stuff like secondary dominants, methods of modulation, etc are what keep pieces in my head. Least reliable is just hoping your fingers remember the way through.

[pianoman1800]

I agree with j_menz about the composer. For instance I played the Chopin Impromptu in A flat when I was 7 or 8 I think. Now, after so many years I can still play it with almost100 % correct notes. I have also played A toccata by an unknown composer when I was 11,and I Don't even remember how the piece sounded!

[indianajo]

There was a 45 year gap between the end of my piano instruction, and my going back to some of those old pieces now.  I memorized with my lower brain in those days, endless practice leading to my forgetting to look at the music is how I felt about it.  When I go back now through the old pieces I knew, most I can get through fairly easily, some I get stuck at certain points, or play obvious mistakes.  I'm taking some of the advice given here about memorization and trying to supplement my "physical" memorization with some appreciation for the chord structure.  Between those days and now I played the guitar some, and learned to appreciate chord structure with the ear in a way that my piano teacher never attempted to instill.
One problem is that I have always used rather different fingering sequences than writtin in the score, because my hands are shaped so differently than north Europeans.  If I didn't write those fingerings down, it is a bit of a chore to remember them.  Some pieces like the JS Bach 2 part Inventions, I have written down new fingerings as I re-acquaint myself with them, then go back to the original fingering automatically when I start getting the piece up to speed.  I don't forget very much ever, I just have trouble sometimes gaining instant access to my memories. It is a shame that my muscles aren't as flexible now as they were then, I have a lot more time to practice now with no school band and no homework to do (except the annual income tax rite, now being procrastinated).