Fifth line
After , which is a good diagonal attachment,
, while not exactly a poor play - it is aggressive, at least - isn't better than Black at a.
A later white play at is rather too easy to make as an invasion.
Plays on the fifth line don't take much secure territory.
On the other hand pushing along the fifth line is no good, because it gives away an extra point each turn, compared with pushing along the fourth line. Here isn't joseki: White at b is the recognised play.
Harpreet: Doesn't this diagram show pushing along the 4th line? Doesn't the number refer to the line of stones that is making territory for one side?
Charles I think one usually understands pushing along the nth line (intransitive) as meaning the same as pushing the opponent along the (n-1)th line (transitive).
Actually there are a few examples in pro games for the diagram above, but that shouldn't confuse one into thinking it's joseki.