On Thu, 01 Feb 2018 23:39:13 +0000, Peter Humphrey wrote:

> > > The Romans were very practical people who didn't waste time or
> > > effort, but got on with the job & built an empire which lasted  500
> > > years .  
> > 
> > built an empire THAT lasted  500 years  
> 
> Nope. Philip was right the first time. They built an empire, and it
> lasted 500 years. Your version implies that they built an empire that
> lasted 500 years, as distinct from one that didn't.

In that case the which should be preceded by a comma, as that part of the
sentence is optional, only adding extra detail. However, here the fact
that the empire lasted 500 years is the point and should be included in
the main statement with that.

The rule of thumb I was taught is that if you can dispense with the
second part without significantly changing the meaning, use which,
otherwise use that. Since the 500 years is crucial, the latter applies.


-- 
Neil Bothwick

You do not need a parachute to skydive. You only need a parachute to
skydive twice.