To enable caps lock on the iPhone (which only has one shift), use the 'Enable caps lock' option in Settings/General/Keyboard. Then you can double-tap shift to turn on caps lock.

I basically never use CL on a regular PC, since it's easier to just hold shift while you type the word most of the time. On an iPhone though, it's actually pretty useful at times.

caps lock on iphone keyboard comes with double clicking the shift arrow. It should turn blue.

I've mapped caps lock to control on my keyboard but I like to use caps lock when I'm typing on a phone. For acronyms and such it is useful and the inability to hold shift on an iphone makes caps lock a better option. However, on a keyboard when it is trivial to hold shift, it is useless.

> on a keyboard when it is trivial to hold shift, it is useless.

I take it you are a touch typist and/or don't suffer from any motor skill problems?