Many problem themes focus on the line effects produced by long-range pieces, such as the battery (lines are opened by discovery), and the Grimshaw (lines are closed due to self-interference).

Another strategic idea based on long-range play is line clearance.

Take two similar line-pieces of the same colour – e.g. two rooks, or a queen and a bishop – and move one of them along a line so that the other can follow along the same line.  The first piece ‘clears’ the line for the second and, in particular, crosses over a critical square which becomes accessible to the following piece.

Such a manoeuvre is called a Bristol clearance.

In Problem 68, the white bishop on the long diagonal makes a series of impure, but thematic, Bristol tries. 

The clearances are not completely pure because the bishop, in crossing c3 to threaten 2.Qc3, also acts as a guard on that square.

The first try, 1.Bd4?, fails to 1…Se4! 1.Be5? provides for 1…Se4 with 2.Qd4, but the bishop in blocking e5 admits the refutation 1…Be1! (since 2.Se5 is now prevented).

Two more tries hinder the white rooks: 1.Bf6? Sb5! (2.Rc6 illegal), and 1.Bg7? Qf6! (2.Rc7 illegal).

Typically in a helpmate with two phases, the play is unified through White playing analogously across the two solutions, and Black doing likewise.

Problem 70, however, achieves harmony in a different way: the matching play across the two solutions is executed not by the same player, but once by White and once by Black.

Part (a) of this twin is solved by 1.Bf5 dxe6 2.Qe4 dxe7, and part (b) by 1.exd6 Rf7 2.exd5 Qe7.

Thus in (a) Black carries out a Bristol and White makes a pair of pawn captures, while in (b) it’s White who carries out a Bristol and Black who makes a pair of pawn captures.

Every move appears as part of a Bristol manoeuvre in the highly intensive 71.

The solutions are 1.Qe4 Rh1 2.Bf5 Qg1 and 1.Rf2 c4 2.R8f5 Rc3.  Both white clearances are pure, including 1…c4 which illustrates the ability of a pawn, on its initial square, to act like a line-piece.

In relation to the point made earlier about how well-constructed problems utilise the pieces fully, notice the functions of the thematic pieces when they don’t play the Bristol moves.  The white pawn guards d3 in one solution, while the white queen guards e4 in the other.