Thinning the Lam in "Allah" — When Is It Thinned and When Emphatic?

The rule:
The lam of the Divine Name "Allah" has two rulings agreed upon by all readers:

1. Emphatic (tafkhim/taghlid): After fatha or damma — examples: "qala Allah" — "rasulu Allah"
2. Thin (tarqiq): After kasra — examples: "billahi" — "lillahi" — "wabillahi"

Origin and reason:
The lam in the Divine Name is originally thin — but becomes emphatic after fatha and damma due to the influence of the preceding emphatic sound. The kasra preserves thinness because it has less emphasis.

Examples from Al-Fatiha:

• "Bismillahi" — sukun on ba + kasra on sin = thin lam
• "Al-hamdu lillahi" — kasra on lam of jar = thin lam
• "Allahi al-rahmani" — fatha before = emphatic in the first "Allah" only

Note for the learner:
This ruling is agreed upon by all ten readers — it is one of the most distinctive features of pronouncing the Divine Name. Ibn Al-Jazari: "Emphatic pronunciation of the Divine Name after fatha and damma, and thin after kasra, is a special Quranic linguistic feature."