making a rhombus from a square

Use the Direct Selection tool (white arrow). Select two opposite corners, switch to the Scale tool, hold down Shift and drag.

If you want your diamond to sit straight, rotate the square 45° before selecting the corners.

Cheryl,

As you must have guessed, Wade means 45 when he says 90; strange habit.

If you want a precise 60 degrees version with just the 4 Anchor Points, you may:

1. Create a triangle with the Polygon Tool and place it with the central bottom Reference Point at X=0, Y=0;

2. Create a rotated copy (Object>Transform>Rotate>Copy and change to Y=0;

3. Pathfinder>Merge.

Well I actually think the easiest ay to do what you want is to draw a square, then rotate it 90º the drag the top and then bottom point a specific distance.

You can do this by selecting the top and bottom anchor points of the rotate square.

Then using the object transform>Scale and give it a percentage