Obligatory prolongation
المد اللازم
al-madd al-lāzim
When a long vowel is followed by a letter that carries no vowel of its own in every circumstance — most famously in the detached letters that open some sūrahs — the lengthening is obligatory and fixed at six counts. All ten agree; this is one of the few places in the whole science with no divergence at all.
Example
الٓمٓ
al-Baqarah 2:1
The ā of لَآ and the ī of مِيمْ are each held six counts by every reader.
By reader
All ten — 6, unanimously