How do I line up fence posts in a straight line?

This may be silly, but it's for a 24' fence, straight line, no corners, not much slope.  I've seen several things, but mostly it is someone putting string from the start post to the end stake and making the other posts just back up to the string.  I understand that.

But, how do I know the string is straight from the first post to the last one 24' away?  I could eyeball it, but Id like to be more accurate.  I'm starting on a small stretch for privacy, but later I want to do other fences.

I think I made one mistake though.  I put in the first post already instead of stakes, which make a 3-4-5 harder, plus it's a long span.

Johann Yorkshire wrote:This may be silly, but it's for a 24' fence, straight line, no corners, not much slope.  I've seen several things, but mostly it is someone putting string from the start post to the end stake and making the other posts just back up to the string.  I understand that.

But, how do I know the string is straight from the first post to the last one 24' away?  I could eyeball it, but Id like to be more accurate.  I'm starting on a small stretch for privacy, but later I want to do other fences.

I think I made one mistake though.  I put in the first post already instead of stakes, which make a 3-4-5 harder, plus it's a long span.

I'm not sure I understand the question.  A piece of string from one post to another will always be straight.  3-4-5 is for making corners square, but you specified no corners, just a straight fence.  Could you try to rephrase the question or add a picture or something to help clarify?

Similar to what Trace suggested, the way we do this is to put a fence post at each end and then put a string on top of each post.

The holes then are dug under the string.

I can't remember though I feel after setting all the posts if any happen to be taller than some, we cut the to the same height.

Is this what you mean?  Did this help?

The string method works well, especially under 50 feet, and up to 100 feet if you pay attention.
The string needs to do three things.
1.) Be very tight between the two stakes/posts defining the location of the fence.
2.) Be as thin/lightweight as possible, to reduce sag from its own weight, and reduce being blown off course by the wind (or work in calm weather). Closer to the ground helps with wind and locating the posts/holes.
3.) Run alongside the posts, but not touching (pushed off course by) any of the posts. (not counting the end posts, which could have the string wrapped to them.)

Yes, you CAN eyeball it. (If it looks straight, it IS straight!)
If you are able to get beyond the end of the fence a bit (preferably from both ends), you can see if the (temporary stakes) are in a line, adjust, and check again until you are satisfied.

Adding to Kenneth's comments, when you place the post in the hole, aligned with the string just above the ground, use a level to get it vertical.
When a few are in place just do a check observation for minor adjustments.
That is how I learned to install 100's of fence posts in the suburbs at 9 ft spacing.

The shortest distance between two points is a straight line.  We can tell it's a straight line by pulling a string tight between the two posts.  Using a string will always make the posts straight in relationship to each other but it may not put it in the right place - that needs other measurements.  

I wonder if the question is "how do I know I got the second post in the right place?"

It sounds like you are measuring it in relation to a third thing like a property line or building or other fence.  For a property line, it is usually surveyed or has a marker to show where the line is.  Use that as a guide when marking out the fence.  

If it's in relationship to another fence or a building, we use that as a guide.  For example, if you are making the new fence at right angles to the old one, we can either use a protractor to measure the angle of the new line from the old - or we can make a triangle out of the string.  Get the right angle triangle math and the old fence is A, the new fence is string B and the diagonal is C.  When B and C strings are tight, we know we have the post in the right location.  

But there are a lot of different ways to measure depending on what the third reference point is.  Can you share more about your situation and maybe someone here can give some tips?

My community typically stretches wire to set the position of the posts. (Like 100 meters at a time.) Then pull it very tight. If I keep the line of posts 1/2" away from the wire, then I worry less about inadvertently pushing the wire out of place. I often sight along the wire to make sure it stays in place.

We did a 600m (2000') farm fence.  I am guessing that you are putting in a solid fence.  If you put in your first and last post then put a wire or string between them.  Divide the total distance by the distance between spans.  This will be on the manufacturer's installation guide in the case of a metal fence and for palings the distance is about 9' outside post to outside post,  and that will give you the number of posts with a bit left over.  With the wire or string taught, there will be a droop but they will be in a straight line between the posts.  Put in the closest to middle post which will halve your problem.  Line up the top of the first and last post then move the middle post up or down until the three post tops are aligned, and all three are vertical.  do the same with a post either side of the centre one and then work from the middle to both ends.  Check after every few posts from one end then the other that the posts are aligned.  Make sure that the string or wire is just brushing the posts.  There can be a tendency to push the string or wire out, causing a curved fence.  The greatest cause of failure when building a fence is not regularly checking.

If in doubt, Youtube is the great teacher of new skills.  Good luck with it .  We would love to see pictures of the finished product.  Consider submitting for a BB.