The verse in the middle of the page

Old Indian textbooks on grammar, medicine and astronomy are strangely short. Some run to a few dozen lines. That was on purpose. Students learned them by heart from a teacher, and short things are easier to memorise. The written copy was a backup.

So the text gives you the bare rules and nothing else. No examples. No exceptions. No help when two rules clash. To use it, you needed a teacher standing there.

When the teacher was no longer there, someone had to write the teaching down. That is a commentary. It became the main way Indian scholars did new work: instead of writing your own book, you wrote a commentary on an old one.

The tradition said as much out loud. The grammarians' first rule of interpretation is that the exact sense of a text is settled by the explanation given of it, and that a text is not faulty just because it leaves you unsure. Read that slowly and you can see what it hands over: whoever writes the explanation decides what the book says.

There are different names for different kinds. A bhashya is the big full one. A vritti is shorter. A tika goes word by word.

Commentaries did more than explain. They preserved. Bhattotpala, an astrologer working in Kashmir in the 900s, quoted older books that no longer exist in any other form. In 2013 a scholar rebuilt one of them, the Parasaratantra, purely out of quotations scattered in three commentaries. There is no manuscript. Only the quotations.

The absence of commentaries tells you something too. The biggest book in Indian astrology, the Brihat Parashara Hora Shastra, has no old commentary at all. The earliest one anybody can find was printed in 1939. Commentaries are wear marks. Where they are missing, the book was not being taught.

Here is the interesting part. Commentators often disagreed with the book they were explaining, and almost never said so. Instead they explained a word in a way that changed its force, or added a missing word, or preferred one version of a line, or said a line was inserted later by somebody else. Change was always presented as recovery. The most famous commentator of all, Mallinatha, promised his readers that everything he wrote came from his source, and then sometimes adopted a reading that earlier scholars had only guessed at.

When two editions of the same old book differ, that is usually two commentators arguing, not two manuscripts.