Walk the word from the first letter, adding each new letter to a running total, and add the running totals together. For אחד: 1, then 1+8 = 9, then 1+8+4 = 13, and the building value is 1+9+13 = 23.
The building value weights a letter by how much of the word it survives, so the first letter counts for the most and the last for the least — the reverse of a plain sum, which weights only magnitude. It is used to bring out the way a word grows as it is read aloud, and the method is treated in the handbooks as a middah of its own rather than as a variant of the standard count.
Nothing in the library points at this entry yet.