Lining up posts with no reference point

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 […]

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