Just in the planning stages of finishing my basement.
When I actually start to locate my 1st wall, what's the correct method to get it/them squared?
Thought I remember someone talking about a 2,3,4 rule?
Is it easier to start with an inside corner or an outside corner first.
Do I take measurements / plumb from an overhead I-beam towards the first wall I plan to work on?
Just trying to plan ahead.
Thanks Dave

You actually build your wall while it is laying on the floor, then you raise it in position and shim to the joists. Plumbing would be accurate enough for walls. Your floor plate should be pressure treated lumber anchored by a nail gun.

Actually it is the 3,4,5 rule. Rather than retyping I just copied this from another site.I'm guessing what your looking for is the old carpentry trick for squaring up perpendicular lines. To use the 3, 4, 5, rule you measure and mark 3 feet from the intersection on one line, and 4 feet on the other line. The measurement diagonally from mark to mark will be exactly 5 feet only if the angle is exactly 90 degrees. BTW This trick can be made more versatile by applying a little more algebra to the Pythagorean theorem. NA^2 + nB^2 = nC^2 is an equivalent equation and the implication is that you can multiply all sides of a 3, 4, 5, triangle by an equal factor. The advantage for this is as follows. Say you are checking the walls of a building 20' x 25' to see if they are square. You then find a factor you can multiply 3 and 4 by that's as large as possible without exceeding the wall length. I this case a factor of 6 works best. So then you measure 18' (3'x6) down the 20' wall and 24' (4'x6) down the 25' wall and then measuring diagonally you will get a measurement of exactly 30' (5'x6) if the walls are at a perfect 90 degree angle. You can also use this trick for objects smaller than 3' x 4' by using 3, 4, and 5 inches instead of feet. For example you want to square a picture frame 22" x 32". The greatest factor in this case is 7. Thus you make a mark at 21" (3" x 7) and at 28" (4" x 7) and the measurement from mark to mark diagonally will be 35" when square. On Square or rectangular objects it's actually easier to check the corners for squareness by making sure that the length from one corner to the diagonally opposite corner equals the length between the remaining two corners. One last trick is if your using the same dimensions for repeated checks you may find it convenient and more accurate to cut a stick the length of the hypotenuse instead of trying to hold and read a kinked measuring tape.