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
(calc-truncate-stack) 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.