I’m sure I’m missing some very easy concept, so forgive me. I’m beginning to build a swingset/clubhouse in the middle of our large backyard. The club house portion needs four 4x4s as the base. How do I ensure that each 4×4 (2 on each side) are lined up even with each other? I have a string level but that only makes sure the string is level “up and down”, and I have no reference point that I can measure from to ensure that they are the same distance from that reference point– we have a large piece of property and this thing is going out in the middle of it. Anyone care to explain this concept to me? :) I’d be grateful.

There is a device known as a post level, which straps around the post, and, has vertical vials in two perpendicular directions, and a vertical plumb vial. You can get one at any building supply store, and, they are not expansive. HF has one for $4.99.

Put up the first post and then the last, pull a string tight between the two post, if you put a string at the top of the post and another at the bottom, you can’t go wrong

Not sure how you mean your question exactly. To place the posts in the correct positions, you put a stake in the ground beyond the outside edge of the structure, on the imaginary outside edge of the first post. Then pull the string tight and measure beyond the imaginary placement of the second post. Put the second stake in the ground and tie the string around the second stake. Do this for all 4 post positions. When you’ve correctly layed out the the strings, the diagonal dimensions will measure the same. (that’s the measurement from outside corner to outside corner) When the two measurements are the same the post positions are square to each other. The stakes are beyond the placement of the post position to keep them out of the way. You want the string on the line of the outside edge of each post. When they are in the ground and the string is tight, it will show the outside edges of the post placements where the strings cross.

I just built one of these structures with my boss. On his the posts didn’t go into the ground. But this still works. You can use marker paint to mark the positions for the corners when you’re happy with the placement. When you’re satisfied with the position take the string down but leave the stakes in the ground. If you need to double check yourself later after you’ve moved the structure in put the string back up to guide you. Be sure the ground is level too. It makes it hard to construct if it’s one of the kits to keep everything plumb and square if the ground wasn’t level to begin with. Remember the diagonal measurement trick. It will tell you while building the thing whether sections are plumb and square or not.