Megillah, Chapter Four, Mishnah Four
This mishnah teaches various rules about reading the Torah in public.
1) One who reads the Torah [in public] may not read les than three verses.
2) And he should not read to the translator more than one verse [at a time], but [if reading from the book of a] prophet [he may read to him] three at a time.
a) If the three verses constitute three separate paragraphs, he must read them [to the translator] one by one.
3) They may skip [from place to place] in a prophet but not in the Torah.
a) How far may he skip [in the prophet]? [Only] so far that the translator will not have stopped [before he finds his place].
Section one: An aliyah may not consist of less than three verses.
Section two: In mishnaic times the spoken language was Aramaic. Many people, perhaps most people, would have had trouble understanding the Torah in its original Hebrew. Therefore, as part of the public reading of the Torah, there was a translator who would translate verse by verse. The reader was to read one verse and then the translator would translate this verse. However, when it came to reading the haftarah from one of the prophets, they allowed the reader to read three verses at a time. They were less exacting on the precision of the haftarah translation than they were for the translation of the Torah. However, if each verse is its own section, then the reader must read each one on its own. This refers to Isaiah 52:3-5 where there are three verses, each considered to be its own section.
Section three: When reading the haftarah, he may skip from place to place so long as he doesnt have to roll the scroll so far that they translator has completed his translation before he gets to the new verse. Today there are many haftarot where we skip from one place in the book to another, or if reading from one of the twelve minor prophets, from one prophet to another. However, when it comes to the Torah it is forbidden to skip around.