diff --git a/packages/core/src/common.ts b/packages/core/src/common.ts
index 99cd092..a06882f 100644
--- a/packages/core/src/common.ts
+++ b/packages/core/src/common.ts
@@ -35,6 +35,16 @@ type GroupPlace = number;
  */
 export type Group = [GroupDigits, GroupPlace];
 
+/**
+ * Index of the group digits in a {@link Group|group}.
+ */
+export const GROUP_DIGITS_INDEX = 0 as const;
+
+/**
+ * Index of the group place in a {@link Group|group}.
+ */
+export const GROUP_PLACE_INDEX = 1 as const;
+
 /**
  * System for stringifying and parsing numbers.
  */
diff --git a/packages/core/src/systems/en-US.ts b/packages/core/src/systems/en-US.ts
index babb9f2..cb4cf0c 100644
--- a/packages/core/src/systems/en-US.ts
+++ b/packages/core/src/systems/en-US.ts
@@ -1,6 +1,11 @@
 // noinspection SpellCheckingInspection
 
-import { Group, InvalidTokenError } from '../common';
+import {
+	Group,
+	GROUP_DIGITS_INDEX,
+	GROUP_PLACE_INDEX,
+	InvalidTokenError,
+} from '../common';
 import { numberToExponential } from '../exponent';
 
 const DECIMAL_POINT = '.' as const;
@@ -287,16 +292,20 @@ const getGroupName = (place: number, shortenMillia: boolean) => {
 			(acc, c, i, cc) => {
 				const firstGroup = acc.at(0);
 				const currentPlace = Math.floor((cc.length - i - 1) / 3);
+				const newGroup = [EMPTY_GROUP_DIGITS, currentPlace] as Group;
 				if (typeof firstGroup === 'undefined') {
-					return [[c, currentPlace]];
+					newGroup[GROUP_DIGITS_INDEX] = c;
+					return [newGroup];
 				}
 
 				if (firstGroup[0].length > 2) {
-					return [[c, currentPlace], ...acc];
+					newGroup[GROUP_DIGITS_INDEX] = c;
+					return [newGroup, ...acc];
 				}
 
+				newGroup[GROUP_DIGITS_INDEX] = c + firstGroup[0];
 				return [
-					[c + firstGroup[0], currentPlace],
+					newGroup,
 					...acc.slice(1),
 				];
 			},
@@ -378,7 +387,7 @@ export const group = (value: string): Group[] => {
 			const currentPlace = Math.floor((exponent - i) / 3);
 			const lastGroup = acc.at(-1) ?? [EMPTY_GROUP_DIGITS, currentPlace];
 			const currentPlaceInGroup = 2 - ((exponent - i) % 3);
-			if (lastGroup[1] === currentPlace) {
+			if (lastGroup[GROUP_PLACE_INDEX] === currentPlace) {
 				const lastGroupDigits = lastGroup[0].split('');
 				lastGroupDigits[currentPlaceInGroup] = c;
 				return [...acc.slice(0, -1) ?? [], [
@@ -584,99 +593,166 @@ interface ParserState {
 	mode: ParseGroupsMode;
 }
 
-export const parseGroups = (tokens: string[]) => {
-	const { groups } = [...tokens, FINAL_TOKEN].reduce<ParserState>(
-		(acc, token) => {
-			const lastGroup = acc.groups.at(-1) ?? [...EMPTY_PLACE];
+const parseThousand = (acc: ParserState, token: string): ParserState => {
+	const lastGroup = acc.groups.at(-1) ?? [...EMPTY_PLACE];
 
-			if (token === THOUSAND || token.endsWith(ILLION_SUFFIX)) {
-				if (acc.mode === ParseGroupsMode.ONES_MODE) {
-					const ones = ONES.findIndex((o) => o === acc.lastToken);
-					if (ones > -1) {
-						lastGroup[0] = `${lastGroup[0].slice(0, 2)}${ones}`;
-					}
-				}
+	if (acc.mode === ParseGroupsMode.ONES_MODE) {
+		const ones = ONES.findIndex((o) => o === acc.lastToken);
+		if (ones > -1) {
+			lastGroup[GROUP_DIGITS_INDEX] = `${lastGroup[GROUP_DIGITS_INDEX].slice(0, 2)}${ones}`;
+		}
+	} else if (acc.mode === ParseGroupsMode.TENS_MODE) {
+		const tens = TENS.findIndex((t) => t === acc.lastToken);
+		if (tens > -1) {
+			lastGroup[GROUP_DIGITS_INDEX] = (
+				`${lastGroup[GROUP_DIGITS_INDEX].slice(0, 1)}${tens}${lastGroup[GROUP_DIGITS_INDEX].slice(2)}`
+			);
+		}
+	}
+
+	// Put the digits in the right place.
+	lastGroup[GROUP_PLACE_INDEX] = getGroupPlaceFromGroupName(token);
+
+	return {
+		...acc,
+		groups: [...acc.groups.slice(0, -1), lastGroup],
+		lastToken: token,
+		mode: ParseGroupsMode.THOUSAND_MODE,
+	};
+};
+
+const parseHundred = (acc: ParserState): ParserState => {
+	const lastGroup = acc.groups.at(-1) ?? [...EMPTY_PLACE];
+	const hundreds = ONES.findIndex((o) => o === acc.lastToken);
+	lastGroup[GROUP_DIGITS_INDEX] = `${hundreds}${lastGroup[GROUP_DIGITS_INDEX].slice(1)}`;
+	return {
+		...acc,
+		groups: [...acc.groups.slice(0, -1), lastGroup],
+		mode: ParseGroupsMode.HUNDRED_MODE,
+	};
+};
+
+const parseFinal = (acc: ParserState): ParserState => {
+	const lastGroup = acc.groups.at(-1) ?? [...EMPTY_PLACE];
+
+	if (acc.mode === ParseGroupsMode.ONES_MODE) {
+		const ones = ONES.findIndex((o) => o === acc.lastToken);
+		if (ones > -1) {
+			lastGroup[GROUP_DIGITS_INDEX] = `${lastGroup[GROUP_DIGITS_INDEX].slice(0, 2)}${ones}`;
+		}
+		// We assume last token without parsed place will always be the smallest
+		lastGroup[GROUP_PLACE_INDEX] = 0;
+		return {
+			...acc,
+			groups: [...acc.groups.slice(0, -1), lastGroup],
+			mode: ParseGroupsMode.DONE,
+		};
+	}
+
+	if (acc.mode === ParseGroupsMode.TENS_MODE) {
+		const tens = TENS.findIndex((o) => o === acc.lastToken);
+		if (tens > -1) {
+			lastGroup[GROUP_DIGITS_INDEX] = `${lastGroup[0].slice(0, 1)}${tens}${lastGroup[GROUP_DIGITS_INDEX].slice(2)}`;
+		}
+		lastGroup[GROUP_PLACE_INDEX] = 0;
+		return {
+			...acc,
+			groups: [...acc.groups.slice(0, -1), lastGroup],
+			mode: ParseGroupsMode.DONE,
+		};
+	}
+
+	return acc;
+};
 
-				lastGroup[1] = getGroupPlaceFromGroupName(token);
+const parseOnes = (acc: ParserState, token: string): ParserState => {
+	if (acc.mode === ParseGroupsMode.THOUSAND_MODE) {
+		// Create next empty place
+		return {
+			...acc,
+			lastToken: token,
+			mode: ParseGroupsMode.ONES_MODE,
+			groups: [...acc.groups, [...EMPTY_PLACE]],
+		};
+	}
+	return {
+		...acc,
+		lastToken: token,
+		mode: ParseGroupsMode.ONES_MODE,
+	};
+};
+
+const parseTenPlusOnes = (acc: ParserState, token: string): ParserState => {
+	const tenPlusOnes = TEN_PLUS_ONES.findIndex((t) => t === token);
+	const lastGroup = acc.groups.at(-1) ?? [...EMPTY_PLACE];
+	if (acc.mode === ParseGroupsMode.THOUSAND_MODE) {
+		return {
+			...acc,
+			lastToken: token,
+			mode: ParseGroupsMode.TEN_PLUS_ONES_MODE,
+			groups: [...acc.groups, [`01${tenPlusOnes}`, lastGroup[GROUP_PLACE_INDEX] - 1]],
+		};
+	}
+
+	lastGroup[GROUP_DIGITS_INDEX] = `${lastGroup[GROUP_DIGITS_INDEX].slice(0, 1)}1${tenPlusOnes}`;
+	return {
+		...acc,
+		lastToken: token,
+		mode: ParseGroupsMode.TEN_PLUS_ONES_MODE,
+		groups: [...acc.groups.slice(0, -1), lastGroup],
+	};
+};
+
+const parseTens = (acc: ParserState, token: string): ParserState => {
+	const tens = TENS.findIndex((t) => t === token);
+	const lastGroup = acc.groups.at(-1) ?? [...EMPTY_PLACE];
+	if (acc.mode === ParseGroupsMode.THOUSAND_MODE) {
+		return {
+			...acc,
+			lastToken: token,
+			mode: ParseGroupsMode.TENS_MODE,
+			groups: [...acc.groups, [`0${tens}0`, lastGroup[GROUP_PLACE_INDEX] - 1]],
+		};
+	}
 
-				return {
-					...acc,
-					groups: [...acc.groups.slice(0, -1), lastGroup],
-					lastToken: token,
-					mode: ParseGroupsMode.THOUSAND_MODE,
-				};
+	lastGroup[GROUP_DIGITS_INDEX] = (
+		`${lastGroup[GROUP_DIGITS_INDEX].slice(0, 1)}${tens}${lastGroup[GROUP_DIGITS_INDEX].slice(2)}`
+	);
+	return {
+		...acc,
+		lastToken: token,
+		mode: ParseGroupsMode.TENS_MODE,
+		groups: [...acc.groups.slice(0, -1), lastGroup],
+	};
+};
+
+export const parseGroups = (tokens: string[]) => {
+	// We add a final token which is an empty string to parse whatever the last non-empty token is.
+	const tokensToParse = [...tokens, FINAL_TOKEN];
+	const { groups } = tokensToParse.reduce<ParserState>(
+		(acc, token) => {
+			if (token === THOUSAND || token.endsWith(ILLION_SUFFIX)) {
+				return parseThousand(acc, token);
 			}
 
-			if (token === HUNDRED) {
-				if (acc.mode === ParseGroupsMode.ONES_MODE) {
-					const hundreds = ONES.findIndex((o) => o === acc.lastToken);
-					lastGroup[0] = `${hundreds}${lastGroup[0].slice(1)}`;
-					return {
-						...acc,
-						groups: [...acc.groups.slice(0, -1), lastGroup],
-						mode: ParseGroupsMode.HUNDRED_MODE,
-					};
-				}
+			if (token === HUNDRED && acc.mode === ParseGroupsMode.ONES_MODE) {
+				return parseHundred(acc);
 			}
 
 			if (token === FINAL_TOKEN) {
-				if (acc.mode === ParseGroupsMode.ONES_MODE) {
-					const ones = ONES.findIndex((o) => o === acc.lastToken);
-					lastGroup[0] = `${lastGroup[0].slice(0, 2)}${ones}`;
-					lastGroup[1] = 0;
-					return {
-						...acc,
-						groups: [...acc.groups.slice(0, -1), lastGroup],
-						mode: ParseGroupsMode.DONE,
-					};
-				}
+				return parseFinal(acc);
 			}
 
 			if (ONES.includes(token as OnesName)) {
-				if (acc.mode === ParseGroupsMode.THOUSAND_MODE) {
-					return {
-						...acc,
-						lastToken: token,
-						mode: ParseGroupsMode.ONES_MODE,
-						groups: [...acc.groups, [...EMPTY_PLACE]],
-					};
-				}
-				return {
-					...acc,
-					lastToken: token,
-					mode: ParseGroupsMode.ONES_MODE,
-				};
+				return parseOnes(acc, token);
 			}
 
-			const tenPlusOnes = TEN_PLUS_ONES.findIndex((t) => t === token);
-			if (tenPlusOnes > -1) {
-				if (acc.mode === ParseGroupsMode.THOUSAND_MODE) {
-					return {
-						...acc,
-						lastToken: token,
-						mode: ParseGroupsMode.ONES_MODE,
-						groups: [...acc.groups, [`01${tenPlusOnes}`, lastGroup[1] - 1]],
-					};
-				}
-
-				lastGroup[0] = `${lastGroup[0].slice(0, 1)}1${tenPlusOnes}`;
-				return {
-					...acc,
-					lastToken: token,
-					mode: ParseGroupsMode.TEN_PLUS_ONES_MODE,
-					groups: [...acc.groups.slice(0, -1), lastGroup],
-				};
+			if (TEN_PLUS_ONES.includes(token as TenPlusOnesName)) {
+				return parseTenPlusOnes(acc, token);
 			}
 
-			const tens = TENS.findIndex((t) => t === token);
-			if (tens > -1) {
-				lastGroup[0] = `${lastGroup[0].slice(0, 1)}${tens}${lastGroup[0].slice(2)}`;
-				return {
-					...acc,
-					lastToken: token,
-					mode: ParseGroupsMode.TENS_MODE,
-					groups: [...acc.groups.slice(0, -1), lastGroup],
-				};
+			if (TENS.includes(token as TensName)) {
+				return parseTens(acc, token);
 			}
 
 			return {
@@ -695,14 +771,14 @@ export const parseGroups = (tokens: string[]) => {
 };
 
 export const combineGroups = (groups: Group[]) => {
-	const places = groups.map((g) => g[1]);
+	const places = groups.map((g) => g[GROUP_PLACE_INDEX]);
 	const maxPlace = Math.max(...places);
 	const minPlace = Math.min(...places);
-	const firstGroup = groups.find((g) => g[1] === maxPlace) ?? [...EMPTY_PLACE];
-	const firstGroupPlace = firstGroup[1];
+	const firstGroup = groups.find((g) => g[GROUP_PLACE_INDEX] === maxPlace) ?? [...EMPTY_PLACE];
+	const firstGroupPlace = firstGroup[GROUP_PLACE_INDEX];
 	const groupsSorted = [];
 	for (let i = maxPlace; i >= minPlace; i -= 1) {
-		const thisGroup = groups.find((g) => g[1] === i) ?? [...EMPTY_PLACE];
+		const thisGroup = groups.find((g) => g[GROUP_PLACE_INDEX] === i) ?? [...EMPTY_PLACE];
 		groupsSorted.push(thisGroup);
 	}
 
diff --git a/packages/core/test/systems/en-US.test.ts b/packages/core/test/systems/en-US.test.ts
index 7aa8313..e01d6ab 100644
--- a/packages/core/test/systems/en-US.test.ts
+++ b/packages/core/test/systems/en-US.test.ts
@@ -263,5 +263,15 @@ describe('numerica', () => {
 		const exp2 = numberToExponential(value2);
 		expect(stringify(value2)).toBe('one trillion five million');
 		expect(parse(stringify(value2))).toBe(exp2);
+
+		const value3 = '12012';
+		const exp3 = numberToExponential(value3);
+		expect(stringify(value3)).toBe('twelve thousand twelve');
+		expect(parse(stringify(value3))).toBe(exp3);
+
+		const value4 = '12020';
+		const exp4 = numberToExponential(value4);
+		expect(stringify(value4)).toBe('twelve thousand twenty');
+		expect(parse(stringify(value4))).toBe(exp4);
 	});
 });