"Excelsior" is one of Sam Loyd

Sam Loyd

Samuel Loyd , born in Philadelphia and raised in New York, was an American chess player, chess composer, puzzle author, and recreational mathematician....

's most famous chess problem

Chess problem

A chess problem, also called a chess composition, is a puzzle set by somebody using chess pieces on a chess board, that presents the solver with a particular task to be achieved. For instance, a position might be given with the instruction that White is to move first, and checkmate Black in two...

s, originally published in London Era in 1861, named after the poem "Excelsior

Excelsior (Longfellow)

Excelsior is a brief poem written and published in 1841 by Henry Wadsworth Longfellow. The famous Sam Loyd chess problem, Excelsior, was named after this poem....

" by Henry Wadsworth Longfellow

Henry Wadsworth Longfellow

Henry Wadsworth Longfellow was an American poet and educator whose works include "Paul Revere's Ride", The Song of Hiawatha, and Evangeline...

. Loyd had a friend who was willing to wager that he could always find the piece which delivered the principal mate of a chess problem. Loyd composed this problem as a joke and bet his friend dinner that he could not pick a piece that didn't give mate in the main line (his friend immediately identified the pawn on b2 as being the least likely to deliver mate), and when the problem was published it was with the stipulation that white mates with "the least likely piece or pawn".

Solution

The solution is as follows:

1.b4!

Threatening 2.Rf5 any 3.Rf1# or 2.Rd5 any 3.Rd1# (with possible prolonging of both by 2...Rc5 3.bxc5 any 4.R mates). White cannot begin with 1. Rf5 because Black's 1.... Rc5 would pin the rook.

Now there are multiple possible half-defences defending only one of threats and one secondary non-thematical defence:

1...Rxc2 2.Nxc2! a2 3.Rd5 (or Rf5) a1Q 4.Nxa1 any 5.R mates.

1...Rc5+ 2.bxc5!

Threatening 3.Rb1#.

2...a2 3.c6!

Again with the same threats as on move one, i.e. 4.Rf5 any 5.Rf1# or 4.Rd5 any 5.Rd1#.

3...Bc7

Because both Rd5 and Rf5 are threatened; the alternative moves 3.... Bf6 and 3.... Bg5 would only defend against one or the other, i.e. they would half-defend. The given move does defend against Rd5 in the sense that 4.Rd5 Bxg3 5.Rd1+ Be1 6. Rdxe1# takes more than the required five moves, and similarly for 4.Rf5 Bf4.

4.cxb7 any 5.bxa8=Q/B#.

The mate is delivered with the pawn which starts on b2.

Any problem which features a pawn moving from its starting square to promotion in the course of the solution is now said to demonstrate the Excelsior theme. Nowadays it is most usually shown in helpmate

Helpmate

A helpmate is a type of chess problem in which both sides cooperate in order to achieve the goal of checkmating Black. In a helpmate in n moves, Black moves first, then White, each side moving n times, to culminate in White's nth move checkmating Black...

s and seriesmover

Seriesmover

A seriesmover is a chess problem in which one side makes a series of legal moves without reply at the end of which the other side makes a single move, giving checkmate or yielding stalemate, depending on the precise stipulation. Checks cannot be given except on the last move of the series...

s.