Harmonizing a melody in SATB means choosing the chords under a tune and placing those chords in four singable parts: soprano, alto, tenor, and bass. The melody is usually the soprano. The bass tells the listener what the chord is. Alto and tenor complete the triad without turning into a second melody that fights the tune.
You can do this with functional harmony and a small vocabulary. Tonic, subdominant, and dominant, used at the right speed, will carry a hymn. Secondary dominants and chromatic color are optional tools, not a requirement for the first draft. This page is the chord choice. The voice-leading page is how those chords connect. The arranging page is how the finished texture fits the choir you rehearse.
What the bass has to say before the inner voices speak
Start by singing the melody and marking the beats that feel like rest. Those beats are your cadences and your half cadences. In a major hymn, a resting beat near the end of a line is often tonic, and the beat just before it is often dominant. Write the bass note for those two beats first: the root of V, then the root of I, or a first inversion if you have a reason to delay the root. If the bass at those two spots is clear, the phrase has a skeleton. If it is not clear, no alto line will save it.
Between the cadences, the bass can move slower than the melody. A melody that wiggles on three chord tones does not need a new chord on every wiggle. Let the bass hold the root, or step to the third of the same chord, while the soprano decorates. Harmonic rhythm is the rate of chord change. Hymns like a chord on the strong beat and sometimes on the next strong beat, not a new root on every quarter. If you change chord every note, the choir will sing vertically, one pile at a time, and the phrase will disappear.
Name the chord out loud as a function, not only as a letter. "This downbeat is tonic, this one is subdominant, this one is dominant" is a sentence you can check. "C, A minor, D minor, G" may be the same progression or a wander, depending on the key. If you cannot say the function, you do not have a progression yet. You have a list of triads. Lists of triads are why a melody can be "harmonized" and still sound like it has no home.
Choose the key, then choose how fast the harmony moves
Find the key from the final note and the notes the tune emphasizes, not from the first chord you happen to like. If the melody ends on C and treats C as rest, try C major or A minor and see which one matches the accidentals you are actually hearing. A tune with a raised seventh wants the major or the harmonic minor that contains it. Do not force a minor melody into a major harmonization because the choir prefers major. Change the key, not the mode, if the problem is only height.
Once the key is chosen, sketch harmonic rhythm on the lyric. Strong syllables get the chord. Weak syllables usually stay inside the chord you already chose, as passing tones or neighbors. "Amazing Grace," which is in the public domain, opens with a pickup that can sit on the dominant and a downbeat that wants the tonic. You do not need a new chord on every note of "a-maz-ing." The downbeat of "grace" can be tonic, and the notes around it can decorate that tonic. That is harmonization. Coloring every syllable is arrangement noise.
Write the bass rhythm before you write the alto. If the bass moves in halves and the soprano moves in quarters, the inner voices can move in quarters only where a passing tone helps. If everyone moves in quarters all the time, the texture is a chorale exercise even when the song is a lullaby. Match the bass speed to the text. A spoken creed can take a chord per strong word. A flowing refrain can hold one chord across a bar and let the melody do the work.
Primary chords, inversions, and the cadence you promise
Live in I, IV, and V until the phrase is boring in the right way. Boring and clear is the right first draft of a hymn. I establishes home. IV, or sometimes ii, prepares the move away. V demands the return. vi is the relative minor, useful when the text softens or when you want a deceptive delay. ii can replace IV when you want a stepwise bass into the dominant. You can write a whole stanza with those chords and be proud of it. Add a secondary dominant only after this skeleton sings.
Inversions are for bass line, not for variety. A first-inversion tonic lets the bass step instead of leaping back to the root. A first-inversion dominant can smooth a passing motion. A second inversion is a special event: the cadential six-four, which sits on the dominant degree in the bass and resolves to a root-position dominant, or a passing six-four between two roots. Do not plant a second-inversion chord on a strong beat and leave it there because the melody note happened to fit. The bass of a six-four sounds like a dissonance that must move. If you did not mean that, use root position.
The cadence is a promise you should keep. Authentic, V to I, is the normal end of a hymn line that feels finished. Approach it from IV or ii so the bass has somewhere to come from. If the melody note on the last beat is the tonic, put the tonic in the bass too and let the other voices complete the triad, doubling the root. If the melody note is the third, the chord is still tonic. Do not invent a new function to avoid doubling. Doubling the root of a root-position triad is the ordinary, good choice. Avoid doubling the leading tone.
Inner voices that complete the chord and still sing
After the bass and the melody are in place, the alto and tenor fill the missing chord tones. A triad has three pitch classes and you have four voices, so someone doubles. Double the root in root position unless a voice-leading reason says otherwise. In first inversion, double a stable tone, often the soprano or the root, and do not double the leading tone. Check that you actually have the third. A chord with a root and a fifth and no third sounds hollow, and choirs notice, especially on a cadence.
Keep the inner parts in a comfortable band inside the check window. That window is soprano C4 to D5, alto F3 to C5, tenor B2 to G4, bass E2 to D4. An alto on F3 and a tenor on B2 are legal and often unkind. Prefer the middle of the range for notes that last. When a chord tone is available in two octaves, pick the one that lets the voice step from where it already is. The nearest chord tone is usually the right one. Reaching for a prettier spacing that makes the tenor leap a sixth is how inner parts stop being singable.
Do not give the alto the melody in disguise. If the alto has a rival tune, the soprano will fight it and the text will blur. Inner parts can have motive, a step and a neighbor that returns, but they should not have lyrics of their own. When you want a real countermelody, write it as a descant for a refrain and thin the other voices so it can be heard. Four independent tunes at once is a motet. A Sunday hymn is not obliged to be a motet.
Secondary dominants, used once and then resolved
A secondary dominant is the dominant of a chord that is not the tonic. The familiar one in a hymn is the dominant of the dominant: in C major, a D major chord that pulls to G. It is useful at a half cadence, when you want the dominant to feel like a temporary home. The raised note, F-sharp in that example, is a leading tone to G. It must resolve up to G in the same voice. If you throw in D major and then wander to A minor, you have spent the tension and not collected it.
Use one secondary dominant in a stanza, not five. Each one asks the choir to tune a note outside the key. A volunteer section can tune one borrowed leading tone if you prepare it by step. A chain of them turns the hymn into a theory exercise and the pitch will slide. The same restraint applies to borrowed iv in a major key, the minor subdominant. It is a strong color under a sober line. It is tiresome when it happens every phrase. Choose the word that deserves the color, and harmonize the rest of the line with the primary chords.
Chromatic passing tones in an inner voice are a different, smaller tool. A tenor who steps C, C-sharp, D between tonic and dominant is singing a line, not changing the chord, as long as the C-sharp is short and resolves up. If that C-sharp lands on the downbeat and just sits there, it is no longer a passing tone. It is a clash. The strong-beat rule is the practical test: a note outside the chord on a beat the choir feels has to be a prepared suspension or it has to be rewritten as a chord tone.
Hear every part, then decide what to export
Play or sing the harmony in this order: bass and melody together, then add alto, then add tenor. If bass and melody already sound wrong, stop and change the chord. Do not bury a bad function under a pretty alto. When all four parts are in, mute one and listen for a line that has no shape. The voice-leading guide tells you how to repair the connection. The parallel-fifths guide tells you how to catch the pairs that fused. Do those repairs before you call the harmonization finished.
A generated draft can take the first pass. Import a melody, including a MusicXML melody from the notation program you already use, and let CantuStudio put the tune in the soprano and write alto, tenor, and bass around it. Then do the listening order above anyway. The draft does not know your text stress, and it does not know that your basses cannot hold the low E. Edit the chord at the cadence yourself if the function is vague. The point of the tool is to give you something to disagree with quickly, not to close the musical question.
When the harmony is the one you will teach, export. MIDI lets a section practice. PDF is the paper copy. MusicXML lets you keep editing in MuseScore, Sibelius, Dorico, or Finale, and that download is Pro. A free account does not get the .musicxml file. Current limits are on the English pricing page. The SATB arrangement app and the choir-director tool describe the same draft from the product side. Bring the reviewed harmonization back to the arranging guide when you are ready to decide texture, breaths, and what happens in the room.