The Sigma series pistols are so coincidental to the competing Glock pistols that Glock sued Smith & Wesson for patent infringement. Smith & Wesson Sigma The case was settled out of court in 1997, with S&W agreeing to make alterations to the Sigma design and pay an undisclosed expanse to Glock.
Smith & Wesson introduced the Sigma classification of recoil-operated, locked-breech semi-auto pistols in 1994 with the Sigma 40, followed by the Sigma 9. Sigma pistols bore so much similarity to Glocks that users could reportedly interchange bountiful parts between Glock and Sigma models.