Well to align mesh objects is more difficult than to align boxels (objects used in-world in edit mode, also named prims). This for the main reason the centre of the mass of each mesh is not always at the exact centre. It depends of the complexity of the mesh object, while a boxel is a kind of “pre-made” shape, keeping the same centre whatever the settings you apply on it.
So there are two solutions :
1 – My preferred one, because I was certainly a watchmaker in a former life, is to align visually, using the Position settings in the Object tab of the edition mode. You rezz two of the objects you want to align, and you focus on the angles of them, making them to match together, hitting the arrows of the x and y Position (meters) settings.
1a – That will help you, you can use a maths method on the same axis : in the edit mode window, at Object tab, you take the x size of the first object, you add it to the x size of the second object, you divide the sum by 2 and add the result to the x position of the first object ; it will give you the x position of the second object. This work for the lengths, for the widths the visual method is to use again.
[(x size of the first object) + (x size of the second object)] / 2 = a sum
sum + x position of the first object = x position of the second object
2 – The second solution is to use the Align button in the edition window.
– Give the right Rotation (degrees) for each of the two objects you want to align :
–> Right click each object and CTRL key move the blue circle until it sounds visually correct.
–> Right click again each object and ensure that the setting of the z Rotation in the Object tab of edit mode is precise : not 89°or 91° but 90°, not 175° or 183° but 180 and so on.
– Visually position the two objects the best as you can.
– Right click and edit the first object
– Hold Shift key and edit the second object
– The two objects are both highlighted.
– Then hit the align button, it gives you big double arrows around the objects.
– Then hold Shift key : the double arrows become single arrows*
– Then play with those single arrows : it should approach each object from each other.
If it leads to a mess, you quit the edit mode by clicking outside the objects, and then to try again.
* The difference between double and single arrows is the single arrows reach the nearest edges while the double ones move the object either to the centrer or the opposite of the nearest object.