Sound logic is Valid_logic that also has true premises. In other words, the premises are true and the conclusion necessarily follows from them, making the conclusion true as well. For example, the following syllogism:
All multiples of ten are multiples of five. (True premise/All A are B)
30 is a multiple of ten. (True premise/C is A)
Therefore, 30 is a multiple of 5. (True conclusion/Therefore, C is B.)