What can cause intonation to change?

[gordon]

If you set the intonation of an acoustic guitar, compensating the saddle, making
sure the other factors are correct etc and it's on when checking the
12th fret harmonic and fretting the 12th fret, what factors can cause
the intonation to change over time?

[Mark McLean]

Gauge of strings, height of the action, tension of the strings (eg an alternate tuning), using a capo, gradual effect of string tension on the geometry of the soundboard/bridge/neck angle.  This is why adjustable saddles on electrics are a good idea, and why acoustics mostly have intonation which is close, but not perfect.

[Greg Mirken]

Mark pretty well covers it!

[Frank Ford]

As strings age, they stretch, and the distribution of mass may become uneven, so when you fret them, you may indeed hear a significant change.  This is especially significant on unwound strings, with nylon and the skinny electric guitar gauges being the most obvious.

 

If you are one of those who stretches strings to "seat" them, you might be exacerbating the problem from the get-go.

 

Rust can accumulate on steel strings and really change intonation.   

 

So, before blaming the change on other issues, be sure to use some new strings. . .

[Kerry Krishna]

I played a 52 Gibson  Southern Jumbo as my main guitar for a 6 years. It was close to perfect in the intonation department, sorta like having a new Taylor that plays in tune everywhere up and down the neck. After 6 years of maybe 20 string changes a year, I was putting new strings on it, and it wouldn't play in tune. Did'nt mater what I did. I went over the whole guitar inside and out, couldn't find the problem, and eventually, I retired the guitar. Last year, I decided to do some work on it, and in the process, found the problem. The entire bridge plate had almost totally come off. The top was distorting the few hundredths of an inch necessary to permanently put it out of tune.