Something is missing from the image below. Can you figure out what that is? Complete the image to the right and then give me the answer which consists of two words.
They are called:
The grids of boxes on the left tell us what should be added in the corresponding boxes on the right (yes, grids - plural...). Because the 10x10 arrangement of boxes is actually...
...four 5x5 arrangements of boxes positioned next to each other. And each 5x5 sub-grid should be interpreted as if they contained letters of the English alphabet, like a Polybius square (a tool used to help encode/decode certain types of cipher). If we do this, the top-left sub-grid would look like this, with the diagonal line in row 2 indicating that I and J share a space (a common device so that all 26 letters fit):
If we focus on the spaces shaded grey, these indicate the letters AEGILNRT - an anagram of TRIANGLE.
...if you look at all four sub-grids, you will see that the top-right one is identical to top-left, and the bottom two are merely reflections of those above them. This suggests we need to add triangles to each of the 4 boxes on the right - the bottom two being reflections of the top two.
If we do this, accommodating the lines already in position, we get the following (labelled) diagram:
What are these? These are commonly-used symbols for the classical elements - our two-word answer!