Invertible counterpoint in Rangsit: Praew swaps two voices and keeps the sustain pedal ready before piano lessons

Praew is eleven and practises on a compact keyboard in a townhouse, with a window that faces a narrow soi. A two-line exercise from a lesson book has been on the stand for a week, the corners already soft from turning, and Praew wants to know why the teacher asked for the same phrase with the hands exchanged instead of only faster.

In Rangsit the afternoon noise from motorbikes can cover a quiet inner voice, so Praew works at a moderate volume, closes the window halfway, and listens for whether the lower line still sounds like a bass after the swap, not only like a decoration moved downward.

Before a single key moves, Praew rests the right foot near the sustain pedal without pressing it, heel on the floor, waiting in case a pedal mark appears. The hands then take the two lines as written, pause, and take them again with the parts exchanged, keeping the same fingering shapes where they still fit.

Two lines that can change places

Invertible counterpoint, sometimes called double counterpoint, is a workshop tool. If the interval of inversion is an octave, a sixth stays a pleasant sound after the swap, while a fifth becomes a fourth and may need care. Composers of two-part inventions used this so a subject could sit above or below its companion line, and a student can borrow the same test even on a short exercise that is not a formal invention.

Praew writes “oct” in the margin of the exercise and numbers the first four intervals. After playing the original, Praew names each interval again in the swapped version and circles any fourth that now sits in a place that wants a different handling, such as a passing tone rather than a held sound.

A short written plan keeps the swapped version from turning into a scramble, and it gives Praew a place to record which fourths need a different touch after the voices change places.

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The swapped version is practised under tempo first. Praew stops if the new bass leaps to a thin note and writes a small hook to remind the ear that this tone now has to carry the harmony. Only after three clean slow runs does the original speed return.

A live comment from a teacher can confirm whether the fourths in the swapped line should stay or move. Praew keeps that question on a small paper tab rather than guessing during the week.

Two-part practice of this kind also supports later three-part reading. Once the ear has heard a line work in both registers, inner voices in a hymn or a simple prelude are easier to follow because they are no longer “the leftover notes”.

On Saturday Praew records one pass of each version on a plain audio note and plays them back-to-back. The memo is for listening, not for sharing. If the swapped bass feels weak, the following practice starts there, with the memo replayed once before any new speed is attempted.

A parent does not need to name intervals. The useful question is whether both versions were played, and whether the small paper tab from the teacher was checked. That is enough to keep the work honest.

Across the narrow soi, a two-line exercise is tried in each direction, interval numbers sit in the margin, and the right foot remains near the pedal while the swapped bass is heard once more.

Frequently asked questions

Q: What is invertible counterpoint? A: Invertible counterpoint is a way of writing two or more voices so the texture still makes sense when one voice is moved above or below another, often by an octave or a twelfth. The intervals have to be chosen so a consonant sound does not turn into an awkward dissonance after the swap.

Q: Where do pianists meet it? A: Two-part inventions and many fugues are built so a subject and a counter-subject can change places. A student can practise the same idea on a short two-line phrase: play it as written, then play it with the hands reversed in register.

Q: What should a beginner watch for? A: Fourths can become fifths, and vice versa, when voices swap. A line that was a good bass may feel thin if it suddenly sits on top. Slow swapping on two bars at a time makes those changes easy to hear.

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