Is American English a Dialect of British English? Two Sibling Dialects, Not Parent-Child

No. American English and British English are sister dialects—both descendants of 17th-century Early Modern English—not a parent-daughter pair. Neither is "the original."

Key Divergences (Not Errors—Just Different Evolutions)

Pronunciation

• Rhoticity: GenAm = rhotic (pronounce all /r/: car, hard, better). RP/standard BrE = non-rhotic (drop post-vocalic /r/: cah, hahd, bettuh). *But*: West Country, Scotland, Ireland are rhotic too.
• TRAP-BATH split: BrE: dance, bath, grass = [ɑː]. GenAm: [æ].
• LOT-CLOTH-THOUGHT: GenAm merges LOT/THOUGHT ([ɑ]); BrE distinguishes ([ɒ] vs [ɔː]).
• Yod-dropping: GenAm drops /j/ after alveolars (new [nu], tune [tun]). BrE keeps ([nju], [tjuːn]).

Vocabulary (Systematic Differences)

Grammar (Minor but Salient)

• Gotten: GenAm past participle of get. BrE: got.
• Present perfect: BrE uses more freely for recent past (I've just eaten). GenAm prefers simple past (I just ate).
• Collective nouns: BrE plural agreement (the team are). GenAm singular (the team is).
• Subjunctive: GenAm mandative subjunctive productive (I suggest he go). BrE often uses should go or indicative.

Spelling

Noah Webster's reforms: -or/-our (color/colour), -er/-re (center/centre), -ize/-ise (realize/realise), -og/-ogue (dialog/dialogue).

Mutual Intelligibility: ~99%

Speakers understand each other effortlessly. Differences are systematic, not random—each variety is internally consistent, codified, and prestigious in its domain.

Call them national standard varieties of World English. Neither owns the other.