Here is a typical chord symbol:

C minor 13th sharp 11th (a good chord for illustration purposes)

This chord has many parts:  the root (C), the modifier (m), the extension (13),  and the alteration (#11).   For a complete discussion of chord symbols please refer to the "About Chords symbols" section of FastChords manual, but for now we are interested in seeing how the chords calculator works.  The following is extracted from the "Entering Chords" section of the "Guided Tour" chapter of the manual.

Step 1: Enter the chord root - Click on the C button

A single note name by itself implies a major chord (major triad for the music theorists).

The root of the chord is marked with a star .  This is maintained as you invert chords (see the "Inverted Chords and Slash Chords" section in the manual).

As soon as one of the possible root notes (A-G) is selected, they all become "fuzzy", meaning they can no longer contribute to the chord name.  At the same time, other buttons become clear.  This system of buttons that become fuzzy or clear is a visual indication as to what's possible in terms of the chords the program knows about.

Step 2: Enter the chord modifier - Click on the "m" button

The "m" in Cm stands for "minor".  So the displayed chord on the keyboard changes from C major to C minor.

Notice some buttons show two symbols.  For example the button for "minor" shows "m" (in red) and "min" (in black).  These are the "reversible buttons".  You can reverse the face of the buttons from one to the other simply by pressing the Shift key as you click the button.   You can also specify the default setting in the program preferences section.

Reversible buttons allow conforming to the particular style of chord symbols found in printed music.

Step 3:  Enter the extension - Click on the 13 button

In addition to the root note marked with a red star, the other notes of the chord are marked either with a red dot , or with a blue dot .

The blue dot notes are the "optional" notes, although all the notes make up the Cm13 chord, if the blue notes are not played, the sound quality of the chord does not change too much.  Which simplifies playing.

For the music theorists, the blue notes are perfect intervals, in this case a perfect 5 and a perfect 11.

Step 4:  Enter the chord alterations - First click on the open parenthesis ( ) button

"Cm13(" is not a complete chord symbol.  The open parenthesis makes it so.  And for as long as the chord symbol remains incomplete, FastChords just displays the root of the chord.

... then click on the sharp (#) button

"Cm13(#" is still an incomplete chords.  Again just the root of the chord is displayed.

... then click on the 11 button.

Now FastChords "sees" a chord symbol it "knows" about -- assuming you would follow with a close parenthesis --, the "Cm13(#11)" chord, so it displays it on the keyboard.

Step 5:  (Optional) finish up - Click on the parenthesis ( ) button.

The final chord.  Notice the title bar of the keyboard window displays the synonyms of the Cm13(#11) chord.  In this case only one: Cm13#11.

That means if the Cm13#11 chord would have been entered, instead of the Cm13(#11) chord, the result would have been identical.

Check the section on "Chord Synonyms" in the manual for more on this subject.  FastChords knows about most of them.

The default octave position on the piano keyboard that FastChords places a chord favors the left hand.  FastChords is used to play from lead sheet music as found in fake books, where songs are written with just chord symbols and melody lines.  The chords are played with the left hand and the melody with the right hand.