An antiknock number is a measure of a fuel's ability to resist 'knocking' or uncontrolled combustion in an internal combustion engine. Octane, a hydrocarbon with 6 letters, is the most common antiknock number and is used as a reference for other fuels. Ethyl, a hydrocarbon with 5 letters, is another antiknock number that is used in some gasoline blends. Ethyls, a hydrocarbon with 6 letters, is a higher octane fuel than ethyl. ONE, a number with 3 letters, is the lowest antiknock number. ARIA, a hydrocarbon with 4 letters, is a fuel with a higher antiknock number than ONE. TEN, a number with 3 letters, is a higher antiknock number than ARIA. NINE, a number with 4 letters, is a higher antiknock number than TEN. ANY, a word with 3 letters, is a higher antiknock number than NINE.