The grandfather-with-siblings chapter is the heart of ar-Rahbiyyah — this matn is the text that, for nine centuries, taught Zayd ibn Thabit’s system to the world. The rules are the same ones taught by at-Tahqiqat al-Fawzan (sibling bundle, chapter 08); here they are presented as the matn’s main position, not as an alternative system.
Why Zayd
The Prophet ﷺ named Zayd ibn Thabit as the one most knowledgeable in fara’idh (chapter 01), and on the grandfather-siblings issue — which the Companions themselves called the thorniest issue — the Shafi’i madhhab (as well as Maliki and the official Hanbali narration) adopts Zayd’s system: the grandfather does not exclude full or paternal half-brothers; he is guaranteed no worse than his best position.
The rules of play
Case 1 — grandfather + siblings only
The grandfather takes the best of two: muqasamah (counted as a sibling: a brother = 2 heads; a sister = 1 head) or 1/3 of the whole estate. The break-even point: muqasamah wins as long as sibling heads are < 4; ties exactly at 4; loses after that.
Case 2 — other fixed-share heirs present
The grandfather takes the best of three: muqasamah over the remainder · 1/3 of the remainder · 1/6 of the whole estate — and never less than 1/6 (via ‘awl if needed). This “1/3 of the remainder” fraction is the origin of 18 and 36 (chapter 05).
A sister alongside the grandfather
Not given a fixed share — she joins the 2:1 muqasamah — except in al-Akdariyyah.
Al-Mu’addah
When a full sister and a paternal half-brother are both present, the full sister counts the paternal half-brother as a head against the grandfather (pressing down the grandfather’s share), then claws back their portion. Classic example: grandfather + 1 full sister + 1 paternal half-brother → 5 heads → grandfather 2/5; the full sister takes up to 1/2 of the estate; the remaining 1/10 then belongs to the paternal half-brother.
Al-Akdariyyah
Husband + mother + grandfather + 1 sister (full or paternal-half). The sister is given a fixed share of 1/2 (the sole exception), the base of 6 is ‘awled to 9, then the shares of the grandfather + sister (1+3 = 4) are pooled and redivided 2:1 → ×3 → base of 27: husband 9, mother 6, grandfather 8, sister 4. This is the only ‘awl and the only fixed share for a sister in the grandfather chapter.
flowchart TB
classDef proses fill:#713f12,stroke:#ca8a04,color:#fef9c3
classDef furud fill:#1e3a8a,stroke:#3b82f6,color:#dbeafe
classDef khas fill:#7f1d1d,stroke:#ef4444,color:#fee2e2
classDef hasil fill:#3b0764,stroke:#a855f7,color:#f3e8ff
A["Grandfather + siblings<br/>(Zayd's system — the matn's position)"]:::khas --> B{"Are other fixed-share heirs present?"}:::proses
B -- no --> C["Best of 2:<br/>muqasamah vs 1/3 of the estate<br/>(breaks even at 4 heads)"]:::furud
B -- yes --> D["Best of 3:<br/>muqasamah · 1/3 of remainder · 1/6 of the estate<br/>(guaranteed minimum 1/6)"]:::furud
D --> E["Al-Akdariyyah:<br/>6 → 'awl 9 → ×3 = 27"]:::khas
C --> F["+ Al-Mu'addah when full and<br/>paternal-half siblings are together"]:::hasil
D --> F
Difference from the previous two bundles — and how to use all three
- This bundle: Zayd’s system = the position the matn holds (and the Shafi’i madhhab — relevant for Indonesian pesantren and fara’idh tradition).
- The al-Fawzan bundle: the same system is taught in full academically, but the author’s tarjih leans toward the grandfather excluding siblings.
- The Ibn Uthaymin bundle: the tarjih of grandfather-like-father is used directly; Zayd’s system is skipped.
For real calculations in Shafi’i-madhhab settings (the Indonesian majority), this chapter’s system is what your examiner/corrector will expect. Chapter 13 weighs when to pick which.
Sources: Matn ar-Rahbiyyah, chapter on al-jadd wal-ikhwah (Shamela 11372); Sharh al-Hazimi (Shamela 36125); map of positions: IslamQA 240582. Full list in 15-references.