Machine Design
A bolt of M 24 × 2 means that

The pitch of the thread is 24 mm and depth is 2 mm
The nominal diameter of bolt is 24 mm and pitch is 2 mm
The effective diameter of bolt is 24 mm and there are 2 threads per cm
Cross-sectional area of the threads is 24 mm²

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Machine Design
A woodruff key is

Capable of tilting in a recess milled out in the shaft
All of these
A tapered key with head at one end
Fitted in pairs at right angles

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