Six round holes, 1/8" (3mm) in diameter. Possible spacings (depending on seeder sprocket combination) from 4-10" apart. Seed rollers have slots or holes to cup seeds, allowing accurate singulation of very small to medium size raw seeds or pelleted seeds, and are compatible with even some larger seeds. Made of a special anti-abrasion plastic to prevent build-up of static electricity. Trial data: Compatible with but won't singulate the following raw-seeded varitieties: Sylvetta (arugula); Mrs. Burns' Lemon, Spicy Bush, Genovese, and Red Rubin (basil); Nabechan (bunching onion); Watercress (cress); Braising Mix, Red Veined Sorrel, Sorrel, Goldberg Golden Purslane, Claytonia, and Redleaf Amaranth (greens); and Scarlet Frill (mustard). Compatible with and will singulate the following raw-seeded varieties: Astro and Surrey (arugula); Hon Tsai Tai, Tokyo Bekana, Red Rain, Red Komatsuna, Mizuna, Tatsoi, and Komatsuna Summerfest (Asian greens); Amethyst Improved, Genovese Compact Improved, and Aroma (basil); Arcadia* and Gypsy (broccoli); Churchill (brussels sprouts); Storage #4 (cabbage); Champion (collards); Cressida/Curly Cress; Ovation Greens Mix, Vitamin Green, Premium Greens Mix, and Elegance Greens Mix (greens); Red Russian and Winterbor (kale); Korridor, Kossak, Winner, and Kolibri kohlrabi; Golden Frill, Ruby Streaks, Garnet Wave, Green Wave, and Red Giant mustard; Black Summer, Win-Win Choi, Red Choi, Joi Choi, and Mei Qing Choi (pac choi); and Scarlet Queen Red Stems and Hakurei (turnip). *Remove felt spring for best results with these varieties.