In season 2 episode 3 "School Hard". Spike says that angel was his sire (for people who dont know what a sire is it's the vampire that made you a vampire). But in the episode fool for love where it shows spike becoming a vampire it is actually drusilla who turns him not angel.
There are two feasible explanations.
1) Dru was Spike's sire, Angel was Dru's sire. This makes Angel his grand-sire. Also, Angel was the one that basically took Spike under his wing, helped craft who Spike eventually became. So although not his true sire, the word becomes more of a metaphor, saying "you created the monster that I now am".
2) Writer's flub. Spike was never meant to be a regular character, but they decided to bring him back as one. Once that happened, they had to really cement Spike's history, and Dru siring Spike worked better for the storyline than Angel being his sire. It was either forgotten that Spike had mentioned Angel as his sire, or was ignored because they did not feel it was a really important detail.
Pick which ever one you want.
The Master sired Darla, Darla sired Angel, Angel sired Dru, Dru sired Spike. I think the previous comment about Spike referring more to the word "sire" as in someone who helped him is correct.
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