Conversation so far:
When did John Mortimer start his legal career?
John Mortimer was called to the Bar (Inner Temple) in 1948.
What firm did John Mortimer work for?
John Mortimer was called to the Bar (Inner Temple).
What type of law did John Mortimer practice?
John Mortimer's early career consisted of testamentary and divorce work.
Did John Mortimer's legal work change later in his career?
On taking silk in 1966, John Mortimer began to undertake work in criminal law.

Question: What is taking silk?
Self-contained rewrite: What does taking silk mean?