What the phrase means
On evenweave and linen, each cross stitch usually spans two threads across and two threads down. Pattern notes may write it as “over two”.
In notes such as “2 over 2,” the first number means two strands of floss and the second means over two fabric threads. “Over two” on its own does not specify the number of floss strands. Similarly, “2 over 1” on Aida can mean two strands over one square.
The size rule
Stitching over two halves the number of stitches per inch: 28-count fabric over two gives 14 stitches per inch, the same as 14-count Aida. Stitching over one thread on the same fabric would double that to 28 and make the design half the size.