Calc uses RPN notation. If you are not familiar with RPN, see RPN Tutorial.
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 (
command allows you to move the ‘.’ marker up and down in the stack;
see 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 Stack and Trail, for descriptions of these and other stack-related commands.