Metric or imperial Most datasheets of SSOP devices I see describe the lead spacing pitch to be 0 65 mm aprox 0 026 For example page 20 Though other national semiconductor SSOP items seem to use 0 025 0 635 mm The difference becomes large quickly 20 0 65 0 635 0 3 mm about half the pitch I don t know what is correct but I d say it should be mentioned