"How To" Assess Nut
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So your playing your Guitar or Bass and on the open string it buzzes, this is typically a nut issue, but how do you really know the nut is the culprit.

I have two methods to confirm the nut is the issue, which method I use is dependant to the strings fitted

Time to assess 5 Minutes
If the strings are not elixirs or nylon strings then we are going to check using a multi meter.
The two test terminals in the air register the number 1.
When I bring the terminals together they register a number, example 0.02, better on my unit, it makes a buzzing noise when this happens
Attaching one of the terminals to the first three strings.
I place the other one onto the fret.
Pluck the string.
If it touches the fret, it will give a reading, on mine it also makes the noise, this nut is too low, the string definetly touched the fret when it vibrated.
The next way is loosen the string
So this little Yamaha Acoustic buzzes on the open Low E string, is the issue the nut, the relief or possibly the saddle, maybe a vibration exists in the top from a loose brace and so forth.
Make sure its tuned to pitch
Most multimeters have a function called continuity, the arrow is pointing to this bit.
Remove it from the nut.
A nice thin straight edge, long enough to sit over the first two frets.

I am just using one of my nut files, a ruler also works.
Place the straight edge over the frets.
Slide it up to the nut, if it glides straight over the nut slot, then the nut is no good, if it hits the nut and stops, the nut is good.
Here on the A string I have repeated the steps, hits the nut so the A string slot is good