Today is the 15th of May.

As the sum of the first five positive integers, 15 is the fifth "triangular number". The sixth triangular number is 21, and the sum and difference of 15 and 21 are also triangular numbers: 6 and 36.

Fifteen also happens to be the number of:

Years of the war between Austria and the Ottoman Turks (from 1591 to 1606);

The Amendment to the US Constitution declaring that the right to vote would not be denied "on account of race, colour or previous condition of servitude";

Blows needed by the executioner to sever the head of Mary Queen of Scots;

Pounds of hydrogen in the average human body;

Medals awarded to Audie Murphy, the most decorated US soldier in World War Two.

Here are some more fifteens to puzzle over:

15 R B in S

"H T TM O B 15" (S, TR)

"E W B F F 15 M" (A W)

15 M O T D M C

15 P in a R U T

(answers tomorrow).