Why does a guitar neck sometimes buzz one day and not the next?
Earlier this week I picked up my Squier Strat and started playing it, but noticed I was getting some serious string buzz, so I put it down. The next day, I picked it up to play again, and there was no string buzz.
Was this magic? No.
On the first day I picked it up, it had been raining for a few hours and I had the window open in the room where the guitar was (I live in Tampa Bay Florida so it wasn't cold or anything). Because of the rain, the air got really thick with moisture, as in it got muggy, and even with air conditioning in the room, there was enough of an environmental change to shift the wood in the guitar neck. As such, the neck flexed a little, and... string buzz.
On the next day, the air and temperature went back to how it normally is (for here anyway), the neck flexed itself back into shape, and... no string buzz.
Is it normal for guitar necks to do this?
For modern guitars with plainsawn guitar necks, yes it is.
There are three ways a board is cut from a log to make a guitar neck with. It's either plainsawn (sometimes known as flatsawn), riftsawn or quartersawn.
Riftsawn is almost never used for electric guitar neck construction for the reason there's a lot of wood waste involved.
Quartersawn is used on more high-end guitar builds, but it's a much more expensive way of cutting boards due to the fact that it does take skill to actually have quartersawn wood cut correctly the first time, and the fact that quartersawn boards do not automatically qualify them as "good". As with any board selecting process, there will be good and bad boards from any log cut. Part of what makes quartersawn more expensive is that the luthier does have to hand-pick a board that not only has the straightest grain but also would make the best choice to form into a guitar neck. The process isn't cheap.
Plainsawn is the fastest, easiest and cheapest way to get a log cut into boards, and that's what the vast majority of electric guitar necks use.
What makes a quartersawn guitar neck so desirable?
A quartersawn board has the straightest possible grain, resulting in a guitar neck that's very stiff.
In luthier's terms, this means you get a neck that's more "stable" compared to plainsawn.
Quartersawn necks can typically handle environmental changes a lot better than plainsawn can. Because quartersawn is more stiff, it resists excessive bowing/flexing. A real-world example of where quartersawn is good is with a gigging guitar. A guitar with a quartersawn neck can handle the hot lights of a stage or outdoor use a lot better than plainsawn can.
How do you recognize a quartersawn neck?
You just look at the grain. Or at least for maple-necked guitars (like Stratocasters and Telecasters) you do since nearly all of them use unpainted maple.
This is quartersawn:
Notice the straight grain lines. Really easy to spot.
This is plainsawn:
Like I said, it's easy to spot the difference between plainsawn and quartersawn.
Is quartersawn always good?
No. There are times when using quartersawn does not make for a proper playing guitar.
For example, certain acoustic guitar builders will use quartersawn for the top of the body as it resists flexing/bowing quite nicely, but never use it for the neck as it can wreck not only the guitar's tonal character but also make it a difficult-to-play instrument.
Where electric guitars are concerned, there is such a thing as having a guitar neck that's too stiff, and with quartersawn you always run that risk.
Seasoned luthiers know how to pick a quartersawn board that has the correct amount of flex to it to avoid the neck-is-too-stiff problem. They know how to pick the right board because they've built many guitars and know which boards work and more importantly which ones do not.
Plainsawn, while cheap, is not "bad" and never has been. Some are quick to say things like, "guitars that have plainsawn necks use nothing but crap lumber". Well, that's not true at all because there are plenty of 20-year-old-or-greater Squier guitars out there, all with plainsawn necks, that never exhibited bad neck problems.
In the end, if you have a guitar with a plainsawn neck (which is probably true since most guitars are made that way) where the neck flexes and causes string buzz one day, don't worry about it. Just wait a day and it should flex back to normal. If it doesn't, a minor truss rod adjustment usually fixes the problem.
An example of a non-Custom-Shop Fender guitar that has a quartersawn neck on it is the Eric Johnson signature Stratocaster. He specifically wanted a quartersawn neck on his model, so that's what it's built with.