Card

Doubling Cube

Mana cost {2}

{3}, {T}: Double the amount of each type of unspent mana you have.

Part of 15 known combos, 2 of them two-card. 3 official rulings.

Bracket

B1 Exhibition — Not in the same deck as its two-card combo partners: Capsize, Filigree Sages
B2 Core — Not in the same deck as its two-card combo partners: Capsize, Filigree Sages
B3 Upgraded — Not in the same deck as its early two-card combo partners: Capsize, Filigree Sages
B4 Optimized — allowed: No restriction beyond the banned list
B5 cEDH — allowed: No restriction beyond the banned list

Two-card combo with Capsize, Filigree Sages (early with Capsize, Filigree Sages)

Bracket: Lotus Vault analysis, with Commander Spellbook flags

15 known combos, 2 of them two-card.

Doubling Cube + Voltaic Key + Rings of Brighthearth
Produces: Infinite mana permanents you control can produce, Infinite untap of artifacts you control
View on Commander Spellbook
Doubling Cube + Capsize
Produces: Infinite artifact ETB, Infinite artifact LTB, Infinite storm count
View on Commander Spellbook
Filigree Sages + Doubling Cube
Produces: Infinite untap of artifacts you control, Infinite colored mana, Infinite colorless mana
View on Commander Spellbook

2004-12-01 · Wizards of the Coast
Doubling Cube's ability is a mana ability.
2004-12-01 · Wizards of the Coast
Any restrictions on the unspent mana aren't copied. For example, if you have {C}{W}{W}{B} with no restrictions on it and {U}{U}{U} that can be used only to cast artifact spells, you'll end up with {C}{C}{W}{W}{W}{W}{B}{B}, {U}{U}{U} that can be used only to cast artifact spells, and {U}{U}{U} that can be used for anything.
2004-12-01 · Wizards of the Coast
The "type" of mana is its color, or lack thereof (if the mana is colorless).

Rulings: Scryfall · Wizards of the Coast