Trimming a leek

When I was younger I used to like to sit on the kitchen bench while Dad was cooking and ask a seemingly endless stream of questions, which he did his best not to get fed up with. However, my favourite thing was to catch him out having contradicted himself, "but last time you said..." 

One of the earliest things I remember Dad teaching me was that the best leeks were those with the most white at their base, 

"Why are you throwing so much of the leek away, when yesterday you told me not to throw away the tops of the tomatoes? Isn't that being wasteful like I was, Daddy?" (I am even annoying myself in repeating this)

"Because the white part is the most flavoursome, Beano; you should only use the white part and a little of the pale green bit for colour. The other bit is chewy and doesn't taste very nice."

I was pleased to see that Wikipedia confirms his advice:

The edible portions of the leek are the white onion base and light green stalk. The dark green portion is usually discarded since it has less flavor. As the leek grows, this part becomes woody and very chewy. 

So when a recipe calls for 'trimmed leeks' this is what it means.