Because fretless intruments have open strings too, Ulf, which can’t be adjusted while playing. Start reading up. 
Only when playing open notes is there. Because they are not equally tempered violin, viola, cello, double bass and trombone should always be played in tune providing the player is competent intonation intonation even if the basic tuning of the instrument is out. A good player should automatically adjust his/her intonation to the other instruments.
Trumpet players have been know to adjust their intonation with the embouchure. Also they tend to sound better on tunes in flat keys rather than C or sharp keys, especially B flat, F or E flat which is worth considering when selecting the key a tune if planning on using brass in the arrangement.
Cheers
Yup, I knew about embouchure and pitch relation to it. I used to play trumpet when I was a little tike lol. Her intonation was pretty consistent though and the adjustment tube brought us where we wanted to be.
I never knew they sound better in flat keys or certain keys in particular. Why is that? Any insight or points to reference reading about this would be greatly appreciated. I think I am going to open up the session and see the key of the different songs and look at the pitch differences.
Ahh The plot thickens. This makes total sense now with what you said, Dave.
Basses, guitars and keyboards are pitched in “C” concert. Trumpet, tenor sax, and clarinet are “Bb” instruments. That means when we play in the key of “C”, those horns must play one pitch up which is their “D”. If we play in “Bb”, they’re playing in their “C”. So a lot of older music is written in either “C”, or flat keys such as “F”, “Bb”, “Eb”, or “Ab”. That would put those horns in “G”, “C”, “F”, or “Bb” respectively. It has nothing to do with intonation but a lot to do with ease of fingering, especially on tenor sax or clarinet.
That’s why you hear a big groan from a sax player when we play in “E” concert. They are playing it in “F#” which is a much more difficult fingering for them.
A good sax or trumpet player is used to it though, and can play well in any key.
http://www.talkbass.com/forum/f30/keys-compatible-trumpets-saxophones-665165/
Yep, easier to play in flat keys but I always felt intonation was better in flat keys for trumpet players. It didn’t matter so much to myself being a trombonist though. Always easier in flat keys though.
Back on topic though. I rarely think solo violin sounds great. 
Much better in an ensemble. Mind you, I remember Glyn posting a gypsy tune he did that did sound great.
Perhaps because they were tuned naturally and not equally tempered, Dave? (Hi, mate, how’s it going, btw!) However good the player, the tuning of the open strings is bound to come into it. These instruments have friendly keys too and people write to them, just as - as you rightly point out - that people write brass parts for the keys that they handle best.
On Sunday I was playing live on guitar with a fiddle player. She sounded fantastic. It did make me wonder about her tuning, which is why I was primed for this question when it came up. I’ve no idea how she tuned up, but I think it would be a good idea to ask her, in the light of this debate. (I also did some recording with a fiddle player years ago. I’m can’t be sure how we tuned him up but there was a tuner and a keyboard to hand so it’s likely we used it. There were no intonation problems with him, that’s for sure.)
Interesting stuff this and rather important so I’ll post back when I’ve asked her.
haha GLYNNNNN! You realize the world is upside down when there is even one boyband on the radio and Glyn … isn’t. We promise to pay for the next X.5 version if they let him back in? 
What a one-of-a-kind mind! 
Sorry, bock on topic!
Yeah Glyn is one MEAN fiddler alright!.. pretty decent ‘Darbukaist’ too!
Oh, for all you great experts in tuning questions I would like to point out that only the lowest open string on any string instrument (“g” for the violin) must necessarily be played whenver that note occurs. All other “open” notes (for the violin d’, a’, e’') would usually be avoided in an intonation-sensitive context and played on one of the lower strings instead.
So unless your violinist is playing g all the time, there is no point (for him) in blaming the perfect fifths of his open strings for anything.
One should also take into consideration the full context of the performance and the recording situation. A string quartet performing is most likely trying to get all their intervals as pure as possible, whereas a string quartet performing with a piano is most likely trying to get the unison lines to be in tune with the piano, perform pure intervals whenver the piano doesn’t play or relate to the bass note of the piano if only the left hand is playing.
You see, intonation is a very delicate and complicated thing. Of course, playing out of tune is a no-go for a professional string player, but any time you get that impression it is very important to look at the broader picture.
You can imagine that this gets even more complicated as soon as layered strings or unison string groups are involved.
So if you have a recording session and you don’t like the intonation of the player(s) there is a whole list of possible measures to be taken, asking them to re-tune their strings to well-tempered fifths is definetely NOT one of them.
What about bad tempered fifths…
Sorry couldn’t resist
We all have our opinions and suggestions and are just trying to help. This kind of high-minded sarcasm is not helpful and reflects badly on you.
This is also true in part for brass players. There are alternative fingerings for many but not all notes. In any case a good player will always play in tune. Good ears are essential to a good player. I’ve come across some technically competent players who unfortunately seem unable to listen to themselves and others while playing and hear the overall sound. A bit like having tunnel vision.
As an aside, guitars and keyboards as we all know use tempered tuning yet wonder how many take this into account when using auto tune? 
Having said that…
we all make mistakes. 
I have to admit that there is one live recording that has the trombone section I was in playing out of tune on sustained note octaves apart. I cringed every time I heard it but I blame one of the others in the section.
Mistakes? Ah, I’ve heard of them, but never bothered to try myself!
Another remark concerning the “sharpness” of open strings,
as you all will probably have noticed by now, natural fifths are not, per se, “sharp”. It depends on the direction.
So, as all string players tune to “a”, the lower open strings will be slightly FLAT as you go down the line.
A quick calculation (two pure fifths down versus to two well-tempered fifths down) shows that the difference is less than half a hertz (195,99 versus 195,56, starting from a’=440Hz). So a very, very skilled player could, should his g be slightly flat compared to whatever else is sounding at the same time, slightly lift the intonation of the open g by pushing down the string basically on the top nut (thereby even being able to apply vibrato).
So, no need to retune anything. Good ears and very well-designed monitoring are the keys.
I will still retune when needed. I’ve heard the differences and the players don’t give me problems. Not every player that has come in here to record needed to be retuned.
One of the most important things I have found is needing near zero foldback latency and a cup off.
Oh and an a note sometimes.
Tuning aside, I’d like to share a technique I’ve used over the years for doubling/tripling a 4 or 8 piece string section.
For example: if an 8 piece section is recorded with the intention to triple them (24 total) I’d set the studio up for 24 players. This means setting up 24 seats and music stands along with a stereo overhead pair and 12 sectional spot mics.
Prior to the first take, I’d seat the players into all 3 positions and set the spot mic levels.
The trick is to leave all the mics on for each take and simply move the players to the new positions per take.
If you have the room and the mics, this technique works much better presenting a more authentic soundfield than if you were to change mic positions between takes.
Another advantage to this (and this is really empirical observation) is that you don’t get a “sameness” to each take when the players are sitting in different positions.
Interesting. I wonder what happen if (practical difficulties aside hehe!) you do the three takes in three different days? Or morning, noon, night? Or day one morning,day 2 noon, day 3 night? Singers would probably sound even more different?
Good point! Providing you have 8 players (and some time with them) at hand and enough rooms and mics for that kind of setup.
I would additionally recommend changing the desk positions for each take (maybe with the exception of the section leaders). The more variety, the better the sound.
This works for choir recording too. I did a similar technique a few months ago to create a bigger choir.