diff --git a/README.md b/README.md
index ad6298d..52abf91 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,46 @@
 # hp
-Command-line calculator
+## Command-line calculator
+
+This is an *rpn* calclulator with a maximum stack size of 5. It has full support for real and complex numbers
+and will calculate summary statistics for a set of reals of real pairs. Spaces are important since they are used
+to distinguish tokens.
+
+## Building
+The source code is written in *Fortran*. The only compilers I've found that will build it are *ifort* and *ifx* from Intel's
+OneApi suite. Makefiles are provides for GNU make (Linux) and nmake (Windows).
+
+```
+hp -h
+
+Command Calculator
+==================
+
+Introduction
+------------
+
+This is a command-line calculator. It supports both real and complex modes, as well
+as degrees/radians selection and precision control. It can be run interactively or as an
+expression parser. This help is deliberately terse to encourage exploration.
+
+-------------------------------------------------------------------------------
+Operators: + - * / ^ ^/x ^x ^2 ^/2 ^3 ^/3 ^*2 ^*10 || ! %
+Constants: pi e g G c two_pi pi_over_2
+Functions: sin cos tan asin acos atan sinh cosh tanh log2 log lg len sq sqrt cb cbrt
+           alog2 alog alog10 gamma ncr npr rem int nint
+ Controls: fix[0-9] clx cl cla 
+    Modes: real complex verbose terse degrees radians
+ Memories: n=0...9  st<n> sw<n> rc<n> cl<n> m<n>+ m<n>- m<n>* m<n>/ msh
+  Complex: ri _ || to_pol to_cart
+  Actions: 1/ -- R r ? > < split drop
+    Stats: { x1 x2 ... } { x1,y1 x2,y2 ... }
+           n ux sx mx lqx uqx uy sy my lqy uqy a b cov corr
+    Quits: q
+
+-------------------------------------------------------------------------------
+
+Examples
+--------
+    hp "fix2 18 2 - 8 2 / * ="                    -> 64.00
+    hp "2 -- complex sqrt ="                      -> (0.00000,-1.414214)
+    hp -c "radians (1,pi_over_2)p ^ * degrees ="  -> (1.000000,180.000000) p
+```