A line in the plane of a triangle is a central line if it passes through two triangle centers. The Encyclopedia of Triangle Centers (ETC) indexes triangle centers as X(1), X(2), X(3), etc. Thus, for example, X(1)X(2) is a central line. In ETC, the entry for X(8) shows that X(8) lies on X(1)X(2) but does not show other centers collinear with those numbered 1, 2 and 8. The list below shows such collinearities.The first line shown is the line at infinity. Among the centers on this line, the two with least indices in ETC are X(30) and X(511), so that the line is X(30)X(511), or simply 30 511. Similarly, the Euler line, listed as the fifth line, is abbreviated as 2 3.
After the first line, the next few are 1 2, then 1 3, then 1 4, then 2 3, . . . ; generally, for k>2, lines i j such that i + j = k are listed in order of increasing k, with i from 1 up to Floor((k - 1)/2), and skipping lines already listed (e.g., line 4 20 is listed as 2 3.)
You can use this page together with ETC to find collinearities. Suppose you want to find centers collinear with X(316) and X(625). First, in ETC find at X(316) and X(625) the line containing both centers: X(2)X(187). Then on this page, scroll to 2 187 to conclude that X(2), X(187), X(316), X(598), X(625), X(1153), X(1383) are collinear.
For a table of named central lines (e.g., orthic axis, Nagel line, Soddy line) with access to a discussion of each, click MathWorld.
1. LINE 30 511 (THE LINE AT INFINITY: aα + bβ + cγ = 0)
30 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 674 680 688 690 696 698 700 702 704 706 708 710 712 714 716 718 720 722 724 726 730 732 734 736 740 742 744 746 752 754 758 760 766 768 772 776 778 780 782 784 786 788 790 792 794 796 802 804 806 808 812 814 816 818 824 826 830 832 834 838 888 891 900 912 916 918 924 926 928 952 971 1154 1499 1503 1510 1912 1938 1946
Number of centers on this line: 1142. LINE 2 3 (EULER LINE: sin 2A sin(B - C) α + sin 2B sin(C - A) β + sin 2C sin(A - B) γ = 0)
2 3 4 5 20 21 22 23 24 25 26 27 28 29 30 140 186 199 235 237 297 376 377 378 379 381 382 383 384 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 546 547 548 549 550 631 632 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 964 1003 1004 1005 1006 1008 1009 1010 1011 1012 1013 1080 1113 1114 1312 1313 1314 1315 1316 1325 1344 1345 1346 1347 1368 1370 1375 1513 1529 1532 1536 1551 1556 1557 1559 1563 1564 1567 1583 1584 1585 1586 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1628 1650 1651 1656 1657 1658 1816 1817 1883 1884 1885 1889 1894 1904 1906 1907 1981 1982 1984 1985 1995 2041 2042 2043 2044 2045 2046 2047 2048 2049 2050 2060 2070 2071 2072 2073 2074 2075 2409
Number of centers on this line: 2223. LINE 3 6 (BROCARD AXIS: bc(b2 - c2)α + ca(c2 - a2)β + ab(a2 - b2)γ = 0)
3 6 15 16 32 39 50 52 58 61 62 182 187 216 284 371 372 386 389 500 511 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 800 970 991 1030 1151 1152 1160 1161 1182 1192 1207 1326 1333 1340 1341 1342 1343 1350 1351 1379 1380 1384 1504 1505 1570 1578 1579 1609 1620 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1805 1806 1970 1983 2011 2012 2019 2020 2021 2022 2024 2025 2026 2027 2031 2032 2033 2034 2035 2036 2037 2038 2055 2076 2080 2088 2092 2193 2220 2245 2271 2278 2305 2420
Number of centers on this line: 1254. LINE 230 232 (ORTHIC AXIS: (cos A)α + (cos B)β + (cos C)γ = 0)
230 231 232 468 523 647 650 676 1637 1886 1990
Number of centers on this line: 115. LINE 44 513 (ANTIORTHIC AXIS: α + β + γ = 0)
44 513 649 650 652 654 656 657 659 661 672 770 798 822 851 896 899 910 1155 1491 1575 1635 1755 2173 2182 2183 2225 2227 2228 2229 2230 2231 2232 2233 2234 2235 2236 2237 2238 2239 2240 2243 2244 2245 2246 2247 2252 2253 2254 2265 2272 2290 2312 2313 2314 2315 2348
Number of centers on this line: 576. LINE 187 237 (LEMOINE AXIS: bcα + caβ + abγ = 0)
187 237 351 512 647 649 663 665 667 669 887 890 902 1055 1495 1960 2223
Number of centers on this line: 177. LINE 325 523 (DE LONGCHAMPS LINE: a3α + b3β + c3γ = 0)
325 523 684 693 850 858 1273 1491
Number of centers on this line: 88. LINE 241 514 (GERGONNE LINE: (1 + cos A)α + (1 + cos B)β + (1 + cos C)γ = 0)
241 514 650 665 905 1323 1375 1465 1638
Number of centers on this line: 99. LINE 1 7
(SODDY LINE: (1+cos A)(cos B - cos C)α + (1+cos B)(cos C - cos A)β + (1+cos C)(cos A - cos B)γ= 0)
1 7 20 77 170 175 176 269 279 347 390 481 482 516 962 990 991 1042 1044 1323 1371 1372 1373 1374 1442 1443 1448 1458 1721 1742 1770 2263 2293
Number of centers on this line: 3310. LINE 1 2 (NAGEL LINE: a(b - c)α + b(c - a)β + c(a - b)γ = 0)
1 2 8 10 42 43 78 145 200 239 306 386 387 498 499 519 551 612 614 869 899 936 938 975 976 978 995 997 1026 1103 1125 1149 1189 1193 1198 1201 1210 1644 1647 1698 1714 1722 1737 1961 1998 1999 2000 2057 2340 2398
Number of centers on this line: 5011. LINE 1 3 (cos B - cos C)α + (cos C - cos A)β + (cos A - cos B)γ = 0
1 3 35 36 40 46 55 56 57 65 165 171 241 260 354 484 517 559 940 942 980 982 986 988 999 1038 1040 1060 1062 1082 1155 1159 1214 1319 1381 1382 1385 1388 1402 1403 1420 1429 1454 1460 1466 1467 1470 1482 1617 1622 1697 1715 1735 1754 1758 1764 1771 1936 2061 2077 2078 2093 2095 2098 2099 2223 2283 2352
Number of centers on this line: 6812. LINE 1 4 (sec B - sec C)α + (sec C - sec A)β + (sec A - sec B)γ = 0
1 4 33 34 73 223 225 226 243 278 388 497 515 581 944 946 948 950 1056 1058 1064 1066 1068 1070 1072 1079 1457 1478 1479 1490 1519 1528 1541 1543 1558 1659 1699 1745 1750 1785 1838 1848 1870 1877 1891 2356
Number of centers on this line: 4613. LINE 1 5 (cos(A-B) - cos(A-C))α + (cos(B-C) - cos(B-A))β + (cos(C-A) - cos(C-B)γ = 0)
1 5 11 12 80 119 355 495 496 952 1317 1387 1411 1421 1483 1484 1718 1807 1837 2006
Number of centers on this line: 2014. LINE 1 6 (b - c)α + (c - a)β + (a - b)γ = 0
1 6 9 37 44 45 72 213 218 219 220 238 392 405 518 611 613 954 956 958 960 984 1001 1023 1083 1100 1104 1107 1108 1124 1191 1203 1212 1279 1335 1386 1449 1453 1616 1642 1713 1723 1724 1728 1736 1743 1757 2087 2176 2256 2257 2273 2284 2300 2323 2324 2329 2427
Number of centers on this line: 5815. LINE 2 6 a(b2- c2)α + b(c2- a2)β + c(a2- b2)γ = 0
2 6 69 81 86 141 183 193 230 298 299 302 303 323 325 333 343 352 385 391 394 395 396 491 492 524 590 597 599 615 940 965 966 992 1007 1150 1184 1185 1211 1213 1270 1271 1611 1613 1641 1648 1654 1812 1963 1991 1992 1993 1994 2063 2086 2106 2238 2287 2303 2407 2421
Number of centers on this line: 6116. LINE 2 7 a(b - c)(b + c - a)α + b(c - a)(c + a - b)β + c(a - b)(a + b - c)γ = 0
2 7 9 57 63 142 144 226 307 329 527 553 579 672 894 908 1025 1400 1423 1445 1447 1652 1653 1708 1741 1944 2002 2094 2285 2406
Number of centers on this line: 3017. LINE 3 7
3 7 943 954
Number of centers on this line: 418. LINE 4 6
4 6 53 217 387 393 397 398 1172 1181 1199 1249 1498 1503 1514 1515 1540 1547 1548 1549 1587 1588 1834 1865 1901 1990 2207 2211 2442
Number of centers on this line: 2919. LINE 3 8
3 8 100 104 944 952 956 1145 1150 1623 1809
Number of centers on this line: 1120. LINE 4 7
4 7 273 342 938 942 971 1119 1426 1439 1446 1535 1827 1847 1849 1850 1863 1876 1892 1893 1895
Number of centers on this line: 2121. LINE 5 6
5 6 68 155 485 486 1352 1353 2165
Number of centers on this line: 922. LINE 3 9
3 9 84 198 268 282 610 936 965 971 1436 1490 1604 1765 1903 2182
Number of centers on this line: 1623. LINE 4 8
4 8 72 92 318 321 329 355 517 962 1530 1824 1828 1829 1851 1867 1868 1871 1872 1878 1900 1902
Number of centers on this line: 2224. LINE 2 11
2 11 55 100 105 120 149 390 497 528 673 1001 1376 1621 2110
Number of centers on this line: 1525. LINE 3 10
3 10 197 355 515 958 993 1324 1376 1626 2217
Number of centers on this line: 1126. LINE 4 9
4 9 10 19 40 71 169 242 281 516 573 966 1276 1277 1512 1542 1544 1706 1753 1766 1826 1839 1842 1855 1861 1869 1890 2183 2270 2333 2345 2354
Number of centers on this line: 3227. LINE 5 8
5 8 1389 1482
Number of centers on this line: 428. LINE 6 7
6 7 294 379 651 673 948 1086 1462 1814
Number of centers on this line: 1029. LINE 2 12
2 12 56 388 529 958 961 1220 1329 1791
Number of centers on this line: 1030. LINE 3 11
3 11 499 1479
Number of centers on this line: 431. LINE 6 8
6 8 594 964 1220 2295 2298 2345
Number of centers on this line: 832. LINE 2 13
2 13 16 530 618 622 624 2004
Number of centers on this line: 833. LINE 3 12
3 12 498 1478
Number of centers on this line: 434. LINE 4 11
4 11 56 104 108 1359 1846
Number of centers on this line: 735. LINE 5 10
5 10 517 946 960 970 1329
Number of centers on this line: 736. LINE 7 8
7 8 65 69 75 85 319 320 322 377 388 518 883 1122 1231 1441 1463 1469 1909
Number of centers on this line: 1937. LINE 1 15
1 15 1251 1276 1652 2306
Number of centers on this line: 638. LINE 2 14
2 14 15 531 617 619 621 623 2005
Number of centers on this line: 939. LINE 4 12
4 12 55 943 1852
Number of centers on this line: 540. LINE 6 10
6 10 1377 1378 2049 2051 2214
Number of centers on this line: 741. LINE 1 16
1 16 1250 1277 1653
Number of centers on this line: 542. LINE 4 13
4 13 61 1523 1524
Number of centers on this line: 543. LINE 8 9
8 9 346 390 391 452 728 950 1334 1697 1731 2082 2136 2269 2321 2325 2347
Number of centers on this line: 1744. LINE 4 14
4 14 62 1522 1525
Number of centers on this line: 545. LINE 5 13
5 13 18 62 395 397
Number of centers on this line: 646. LINE 7 11
7 11 658 1156 1996
Number of centers on this line: 547. LINE 2 17
2 17 62 532 627 629 634 636
Number of centers on this line: 848. LINE 4 15
4 15 17 1527 1545
Number of centers on this line: 549. LINE 5 14
5 14 17 61 396 398
Number of centers on this line: 650. LINE 6 13
6 13 14 113 115 265 381 399 542 1625 1879 1989 2341 2437
Number of centers on this line: 1451. LINE 7 12
7 12 1268 1788
Number of centers on this line: 452. LINE 8 11
8 11 1320 1329 2098
Number of centers on this line: 553. LINE 1 19
1 19 28 48 240 284 380 610 1172 1474 1582 1610 1630 1781 1844 1953 1955 1973 2172 2173 2294 2301 2302 2303 2304 2332 2360
Number of centers on this line: 2754. LINE 2 18
2 18 61 533 628 630 633 635
Number of centers on this line: 855. LINE 4 16
4 16 18 1526 1546
Number of centers on this line: 556. LINE 2 19
2 19 534 1848 2339
Number of centers on this line: 557. LINE 10 11
10 11 121 392 1145 1682
Number of centers on this line: 658. LINE 1 21
1 21 31 38 47 58 63 81 191 255 283 595 758 774 846 896 920 968 993 1046 1468 1496 1497 1580 1612 1621 1707 1725 1749 1780 1822 1823 1923 1931 1954 1959 1962 2292 2328
Number of centers on this line: 3959. LINE 10 12
10 12 65 72 181 209 210 226 442 758 1211 1215
Number of centers on this line: 1260. LINE 6 17
6 17 18 195 231 233 1209 1506 1656 2439
Number of centers on this line: 1061. LINE 1 24
1 24 1061 1452
Number of centers on this line: 462. LINE 6 19
6 19 34 65 221 478 607 608 1409 1456 1829 1841 1876 1880 2082 2182 2202 2261 2262 2263 2264 2285 2288 2331 2362 2443
Number of centers on this line: 2663. LINE 1 25
1 25 1036 1039 1763 1773 1829 1905
Number of centers on this line: 864. LINE 6 21
6 21 941 1778
Number of centers on this line: 465. LINE 6 22
6 22 251 1176 1180 1501
Number of centers on this line: 666. LINE 7 21
7 21 56 86 348 1001 1014 1284 1434 1444 1804
Number of centers on this line: 1167. LINE 8 20
8 20 40 63 84 189 271 280 515 1158 1782 2357
Number of centers on this line: 1268. LINE 13 15
13 15 30 396
Number of centers on this line: 469. LINE 6 23
6 23 353 1383
Number of centers on this line: 470. LINE 8 21
8 21 55 333 345 958 1043 1259 1792
Number of centers on this line: 971. LINE 1 29
1 29 92 158 820 1047 1096 1784 1895 1896
Number of centers on this line: 1072. LINE 6 24
6 24 54 973
Number of centers on this line: 473. LINE 9 21
9 21 41 78 284 1819 2268 2287 2289 2327
Number of centers on this line: 1074. LINE 14 16
14 16 30 395
Number of centers on this line: 475. LINE 1 30
1 30 79 500 554 1081 1464 1717 1836
Number of centers on this line: 976. LINE 6 25
6 25 51 154 159 161 184 206 232 1194 1474 1495 1660 1843 1915 1971 1974 2194 2203 2299 2445
Number of centers on this line: 2177. LINE 10 21
10 21 35 80 100 759 1283 1793
Number of centers on this line: 878. LINE 1 32
1 32 172 609 760 985 1572 1759 1914 2241 2242 2243 2344
Number of centers on this line: 1379. LINE 2 31
2 31 171 238 748 750 752 985 2239
Number of centers on this line: 980. LINE 15 18
15 18 140 398
Number of centers on this line: 481. LINE 16 17
16 17 140 397
Number of centers on this line: 482. LINE 2 32
2 32 83 251 315 626 754 1078 1369 1506 1627 1799 2240
Number of centers on this line: 1383. LINE 3 31
3 31 601 602 1036 1064 1193 1245 1472 2221
Number of centers on this line: 1084. LINE 7 27
7 27 81 222 278 608 1172 1396
Number of centers on this line: 8