The Sacred Tile rules are used only in Japanese style Mah Jongg. Once you have discarded a tile you can not win by calling for that same tile. You can however win by drawing the tile yourself. The rule is important because this style of Mah Jongg is very defensive and your discarded tiles are used by the other players to determine what might be safe for them to discard and they give clues as to what type of hand you may be playing.
The rule becomes a bit more complex however when you are waiting on either end of a sequence. If you have discarded one tile from one of the ends you can not win by calling the other end tile. Both tiles become sacred tiles. This is often referred to as the 1-4-7 rule.
In this case a 2 crack or a 5 crack will give you a complete sequence and that is all you need to win. But early on you discarded the 2 crack so now both tiles are considered sacred and you must either draw it yourself or change your hand to win another way.
Another example to show some of the complexities follows here: You have already discarded the one dot. You are waiting to win on the end of a sequence. the one dot is sacred and you can not call it to win. You can win however on a 4 dot or a 7 dot as they would complete the 5-6 dot sequence.
If however you had discarded the 4 dot previously in the above hand then according to the 1-4-7 rule all three tiles the one dot, the 4 dot and the 7 dot are all considered sacred and you must self-draw the winning tile.
Sacred Tiling is especially dangerous when you are working a hand with a lot of sequences all in the same suit. You must pay careful attention to your discards as you work the sets of sequences. Not only is the rule 1-4-7 but some also refer to 2-5-8 and 3-6-9 because when working with multiple sequences you can see how easy a set can be completed by either of these tiles.
