Paul Hindemith's Rule 20 in his The craft of musical composition: Book 2: Exercises in two-part writing has two paragraphs:
If the last tone of the model is approached from a fifth above or a fourth below, [then] the [second] voice must move into its last tone by a [major or minor second] progression. Conversely, the [second] voice may use the final progressions given in Rule 15e only if the model moves into the last tone by a [major or minor second] progression.
If we are certain that a [major or minor second] step may be used in the [second] voice between the next-to-the-last and the last tone, then the closing tone of the model may be approached by a skip of a fourth from above or by the skip of a fifth, a major third, or a minor third from below. ... Condensed and in a simpler form: If one of the two voices reaches its final tone by skip, [then] the other voice must reach its final tone by step.
For reference, Rule 15e reads:
In addition to the approaches to the last tone permitted by Rule 5, [the second voice may approach the last tone] also [by] the skip of a fifth from below and that of a fourth from above.
I have two questions:
1 Is the following a fair paraphrase of Paragraph One? If the last interval of the model is a downward fifth or an upward fourth, then the second voice's last interval must be a step. If the second voice's last interval is an upward fifth or a downward fourth, then the model's last interval must be a step.
2 Could someone please explain or paraphrase the entirety of Paragraph Two? Namely:
(a) How do we know that a major or minor second may be used as the final interval of the second voice? Are the relevant rules to be obeyed just these: Rule 3, if the second voice is the lower voice; Rule 18; and part of the first paragraph of Rule 20, namely, the model's last interval must be a downward fifth or an upward fourth?
(b) When Hindemith says that ``then the closing tone of the model may be approached by a skip of a fourth from above or by the skip of a fifth, a major third, or a minor third from below,'' does he mean that these intervals are permitted, or does he mean that they are in this case obligatory?
(c) By the final sentence (``Condensed and in simpler form: ...'') does Hindemith mean to replace both paragraphs of Rule 20 or merely the second one?