1. grid of cells with external borders
2. label rows with integers
3. label columns with letters
4. cells can hold strings with styles, typed numbers, expressions, etc.
5. expressions cannot contain strings
6. currency type has fp arithmetic properties
7. expressions cannot contain cycles
8. evaluation and display is continuous
9. main commands are: create, read from file, save to file, select arbitrary subgrid, modify cell or selection, print, etc.