Well, technically, three flat stones. Two flat-appearing stones can match (like two stacked Pringles chips) and still not be flat and thus not match a third true flat reference. Thus:
"When two plates are not flat but still match, one will not match the third. By continually lapping or scraping the high points of their contact until all three show perfect bearing when intercompared, three flat planes are created" [1]
[1] Foundations of Mechanical Accuracy by Wayne R. Moore c. 1970
"When two plates are not flat but still match, one will not match the third. By continually lapping or scraping the high points of their contact until all three show perfect bearing when intercompared, three flat planes are created" [1]
[1] Foundations of Mechanical Accuracy by Wayne R. Moore c. 1970
(An absolutely fantastic read)