Calc uses RPN notation. If you are not familar with RPN, see section RPN Calculations and the Stack.
To add the numbers 1 and 2 in Calc you would type the keys: 1 RET 2 +. (RET corresponds to the ENTER key on most calculators.) The first three keystrokes "push" the numbers 1 and 2 onto the stack. The + key always "pops" the top two numbers from the stack, adds them, and pushes the result (3) back onto the stack. This number is ready for further calculations: 5 - pushes 5 onto the stack, then pops the 3 and 5, subtracts them, and pushes the result (-2).
Note that the "top" of the stack actually appears at the bottom
of the buffer. A line containing a single `.' character signifies
the end of the buffer; Calculator commands operate on the number(s)
directly above this line. The d t (calc-truncate-stack)
command allows you to move the `.' marker up and down in the stack;
see section Truncating the Stack.
Stack elements are numbered consecutively, with number 1 being the top of
the stack. These line numbers are ordinarily displayed on the lefthand side
of the window. The d l (calc-line-numbering) command controls
whether these numbers appear. (Line numbers may be turned off since they
slow the Calculator down a bit and also clutter the display.)
The unshifted letter o (calc-realign) command repositions
the cursor to its top-of-stack "home" position. It also undoes any
horizontal scrolling in the window. If you give it a numeric prefix
argument, it instead moves the cursor to the specified stack element.
The RET (or equivalent SPC) key is only required to separate two consecutive numbers. (After all, if you typed 1 2 by themselves the Calculator would enter the number 12.) If you press RET or SPC not right after typing a number, the key duplicates the number on the top of the stack. RET * is thus a handy way to square a number.
The DEL key pops and throws away the top number on the stack. The TAB key swaps the top two objects on the stack. See section Stack and Trail Commands, for descriptions of these and other stack-related commands.