Apologies for adding to a seemingly finished thread but I was just reading through and thought to add a couple of things to the reasons already noted why a 5th is perhaps the most difficult interval to play in tune on a violin in case anybody is interested...
When you play a note on a string, by pressing it down onto the fingerboad you stretch the string.
Different makes/models of string are affected in differing amounts by this stretch - for example, a high tension steel core string will stretch at a different rate (in terms of pitch change) than a low tension perlon core, aluminium wound string. That is, a 2mm stretch of one string will result in a different amount of pitch change to a 2mm stretch of a different string.
Violinsts (and other string players) use a mix of strings for a huge number of reasons. With my violin, for example, if I use the same tension and structure string for all of them, the D string doesn't speak very clearly. If I use a higher tension G then the D speaks more clearly but that affects the tone of the A string so I also need to use a different type of A string to get a closer match in tone across the instrument.
The thickness and tension of strings obviously also has an impact on the pitch change to stretch ratio. An E string will change in pitch more than the G string when stretched the same amount.
Added to this, the height of adjacent strings above the fingerboard will most likely be slightly different even on the most well set up of instruments. (This is usually at least in part a deliberate thing for various boring reasons).
So not only do you have to contend with trying to place a single finger equally on 2 strings when the angle of the finger makes that difficult to do so, you must also compensate for differing effects of pitch change due to string stretch and different amounts of physical string stretch on those strings.
This problem is far less on a cello or double bass due to the length of the strings but is a real issue for violinists.
If I play a 5th and change the amount of pressure on each string the pitch change can be up to +/- a semitone on each string without moving my finger along the string, just by changing how much I'm pressing on one string compared to the other.
Another reason to avoid them for a single player is to do with tone quality.
When you place a finger on the string it should be curled over with the tip of the finger pressing onto the string.
The tip of the finger pressed firmly creates a fairly hard edge and produces the clearest sound because it allows the string to vibrate relatively unimpeded. If you place a finger either not firmly, or use the pad rather than the tip of the finger, you will get a rubbish sound which will not sustain because the fuzzy edge acts like a blanket on the string and stops it vibrating efficiently.
Now if you imagine double stopping - using 2 fingers means you can get 2 clearly defined notes because each finger can play in its tip. Using a single finger across 2 strings (like when playing a 5th) means you can't get that good tone because by necessity you are having to use more of the pad than the tip of the finger.
As you move out of 1st position, this problem increases because the strings physically get further apart and you have to use a 'flatter' finger more and more.
