Abel Tassin d'Alonne[a] (The Hague, 1646 – The Hague, 24 October 1723[2]) was a Dutch courtier and diplomat, who was private secretary[b] of Mary II of England, William III of England, and Anthonie Heinsius, and played a secret role as the chief of Heinsius' Cabinet noir and as a cryptographer of note.
[c] His mother later married a Walloon officer in the service of the Dutch Republic by the name of Charles Tassin d'Alonne, and Abel took his name.
[6] After Prince William III married Princess Mary, the daughter of James, Duke of York, the brother of king Charles II of England in 1677, d'Alonne became her private secretary.
When William in his turn died in 1702, d'Alonne became the private secretary of the Grand pensionary Anthonie Heinsius of the Dutch Republic.
As such he also became involved in the latter's secret diplomacy (as he had been in William's service) and he took over Heinsius' Cabinet noir, the center of Dutch espionage during the War of the Spanish Succession.
The first suspected instance was when the French ambassador to the Dutch Republic the Comte d'Avaux got himself involved in a scandal in 1684, when he wrote an indiscreet letter to pro-French regenten in Amsterdam.