What is the Family Core in Blue Prince?

A four-number cipher built from the MCCXIII Roman numerals on the family coat of arms. It reduces via 1000 / 200 * 11 − 2 = 53, and the sum of its digits (5+3=8) tells you that Key 8 opens safe deposit box 53 inside the Vault.

Where do the Roman numerals MCCXIII appear?

On the coat of arms found in both the Wine Cellar and the Master Bedroom. Two Blue Tents notes (in the Wine Cellar and Mechanarium) explain how to split the numerals into four numbers.

How many Vault Keys are there?

Four numbered Vault Keys: 149, 233, 304, and 370. Each opens its own numbered safe deposit box. Plus the derived Key 8 for box 53.

Where can I find Vault Keys?

Dig spots (Shovel or Detector Shovel), the Throne Room (can spawn 149, 233, 370), the Bunk Room (149, 304 normally; all four when drafted from a Cloister of Mila), the Locker Room (149 and/or 233 in lockers), and the Music Room (all four under specific conditions).

The Vault — Family Core Cipher & Vault Keys

Updated May 25, 2026 · 9 min read · Real puzzle mechanics from wiki

The Vault is a Dead End room that contains 40 Gold on the floor and a wall of numbered safe deposit boxes. The puzzle has two layers: the Family Core cipher, derived from the Roman numerals MCCXIII on the family coat of arms, which tells you which deposit box matters most; and the Vault Keys (numbered 149, 233, 304, 370) collected elsewhere in the mansion that open the other boxes. This page covers both layers, where to find every Vault Key, and what's inside each box.

Quick reference: MCCXIII → split into four numbers per the Blue Tents notes → reduce 1000 / 200 × 11 − 2 = 53. Sum of digits 5+3=8 → Key 8 unlocks deposit box 53. The other four Vault Keys (149, 233, 304, 370) come from dig spots, the Throne Room, Bunk Room, Locker Room, or Music Room.

Where the MCCXIII Comes From

The Roman numerals MCCXIII are stamped on the family coat of arms. You can see the same coat of arms in two early-game rooms:

Wine Cellar. Mounted above the rack. A Blue Tents note in the same room explains that the numerals encode four distinct numbers.
Master Bedroom. Hangs over the fireplace.

A second Blue Tents note found in the Mechanarium clarifies the split: the first Roman numeral is a single number, and each of the following three numbers is built from two consecutive numerals.

Reducing the Family Core to a Number

Split MCCXIII as M · CC · XI · II. That gives the four-number sequence 1000, 200, 11, 2. Apply the reduction described in the notes — multiply, divide, and subtract in sequence to collapse to a single number:

1000 ÷ 200 × 11 − 2 = 53

The Vault has a Blue Tents note inside that reads: "the Family Core is unlocked only by the sum of its digits." So:

5 + 3 = 8

The Family Core deposit box is number 53, and the key that opens it is Key 8 — picked up from a separate puzzle chain (see Picture Puzzle for the chest that yields Key 8).

The Four Numbered Vault Keys

Beyond the Family Core box, the Vault has additional numbered deposit boxes opened by four Vault Keys. Each key spawns in several possible rooms — most runs only see one or two on any given day.

Vault Key	Likely sources
149	Throne Room, Bunk Room, Locker Room (lockers), Music Room (if 3 of 5 major keys held), Dig Spots
233	Throne Room, Locker Room (lockers), Music Room (conditional), Dig Spots
304	Bunk Room, Music Room (conditional), Dig Spots
370	Throne Room, Music Room (conditional), Dig Spots

Bunk Room from a Cloister of Mila: if you draft the Bunk Room from inside the Cloister of Mila (instead of from the regular pool), all four Vault Keys can spawn there. This is the fastest way to clear the Vault in a single run.

Dig Spots — the Backup Source

Vault Keys also spawn in dig spots around the estate. You need:

A Shovel from the Workshop, or the Detector Shovel (Shovel + Metal Detector combine).
A visible dig-spot marker (a small mound of earth, usually near outer-area paths).

The Detector Shovel reveals more dig spots per day and is the recommended tool once you have the parts.

Inside the Vault

The Vault is one of the few "Dead End" rooms in the draft pool — only one door, leading back. What you'll find:

40 Gold on the floor, free pick-up on every entry.
The numbered deposit boxes along the back wall. Box numbers map to the Vault Keys you've collected, plus box 53 for the Family Core.
The Blue Tents note on a side table — confirms the "sum of digits" rule for the Family Core.

Box contents vary — most contain a mix of gems, gold, and one-time items. The Family Core box (53) is the lore payoff: it holds a document tied to the Throne Room / Crown of Fenn Aries route (see All Endings).

Vault FAQ

Is the Vault code 4558? No. That's a fan myth that propagated through early guides. The real Vault has no single 4-digit dial code — it has numbered safe deposit boxes, each opened by a specific Vault Key. The Family Core puzzle (MCCXIII → 53 → Key 8) is the cipher most players mean when they ask about the "Vault code."

Do I need every Vault Key for any ending? No. Specific keys are tied to specific deposit-box rewards. The Family Core box (53) is the one most relevant to the Throne Room ending; the others are gem/gold loot.

Does the Family Core math reset per save? No. MCCXIII reduces to 53 on every save. The cipher is fixed.

What about the Boudoir safe? Different puzzle. The Boudoir contains a separate safe with a 4-digit date code (1225 — December 25 Christmas scene). See the Picture Puzzle page for the eight dated safes.

Puzzle

Picture Puzzle

The 44-letter cipher and the 8 dated safes (including Boudoir 1225).

Read solution

Endgame

Room 46

Antechamber, Basement Key, and the Inner Sanctum chain.

Walkthrough