Suji
Read the numbers across. 
Suji 「筋」 involves the general inference of discarded tiles, when determining safe tiles. The term alone points towards the principle of utilizing the "mahjong intervals". This applies to the numbered suits: souzu, pinzu, and manzu. While these three suits are numbered 1 through 9, the numbers may be arranged in a square fashion to determine the "intervals". The purpose is to deduce and/or determine tiles waiting via open waits. The intervals themselves are based on the threetile sequential groupings. Tiles along the intervals mark needed tiles to complete particular sequences. Reading the numbers to the right across, the three main intervals are 147, 258, and 369. Naturally, numbers within each interval can be paired such as 14 and 25. Other references to suji are available, such as matagi suji and ura suji.
Intervals
The intervals for suji apply to any of the numbered suits. Nine types of intervals are available. For these examples, each of the three suits are used; but they apply to all of them. Four tile patterns particularly utilize these intervals: ryanmen, ryanmenten, nobetan, and sanmentan. In the following table, only the first two are examined.
Middle tiles  Waiting for  Interval name  Completion 

Twosided, or Ryanmen  
14, or Iisuu 「イースー」  or  
25, or Ryanuu 「リャンウー」  or  
36, or Saburou 「サブロー」  or  
47, or Suuchii 「スーチー」  or  
58, or Uuppaa 「ウッパー」  or  
69, or Roukyuu 「ローキュー」  or  
Threesided, or Ryanmenten  
147, or Iisuuchii 「イースーチー」  and OR and OR and  
258, or Ryanuuppaa 「リャンウッパー」  and OR and OR and  
369, or Saburoukyuu 「サブローキュー」  and OR and OR and 
The left column shows tiles that may appear in a player's hand, while the middle tiles show the tiles needed in order to complete a needed tile group. The recognition makes it helpful to identify waiting tiles by association. In the case of these waiting patterns, if a hand is waiting for one tile, it is likely waiting for another tile in the interval. Even if these take on Japanese names, they are simply the numbers. Finally, the end result shows the the waiting tiles as the completed tile grouping(s).
Nakasuji
Nakasuji 「中筋」 is literally the "middle suji". In other words, given the three different intervals, they are the middle numbers of 4, 5, and 6.
Defense
Defending using suji essentially presumes a player to be utilizing either the wait pattern of ryanmen, ryanmenten, nobetan, sanmentan, or some other variation. Under the rule of furiten, if a player discarded a waiting tile, then the player's ability to call "ron" is disabled. Therefore, certain tile discards may rule out certain mahjong intervals.
For example, take the 147 interval. If a 4 is discarded, this makes 1 and 7 safer, since if the opponent has a 23 or 56 as their final shape, they would be furiten on the 4. Note that a 7 being present does not necessarily make a 4 safe, as they could still have a 23. Therefore, both a 1 and 7 need to be present in order for 4 to be safer. In such a situation, the 4 is referred to being nakasuji. The same applies to the other intervals.
Of course, one should not be necessarily dependent on these intervals alone, as some waiting patterns are immune to suji, such as a kanchan or shanpon. A toitoi hand is immune to suji.
Because of this, not all suji tiles are equally safe. Suji terminals (1 and 9) are the safest, as the only shapes that can be waiting on them are a tanki or shanpon. Nakasuji (4, 5, and 6) are the next safest, as they add the possibility of a kanchan. Suji 2 and 8 are roughly equivalent to nakasuji. Finally, suji 3 and 7 are the most dangerous (though still reasonably safe), as they add the chance of a penchan wait.
This doesn't apply to the riichi tile. The suji of the riichi tile is generally agreed to be dangerous, thanks to the strength of ryankan shapes. See the Offense section for more information.
28 Suji vs 456 Nakasuji Safety
There are a few considerations about the relative safety of 28 suji and 456 nakasuji tiles.
In a game without red fives, the 456 tiles could be seen as safer. This is because to have a kanchan on a middle tile, for example, a 4, they would have had a 135 shape in their hand. It's generally seen as better to wait with the 13 shape, since the 4 is more useful and less likely to be discarded, so a middle tile kanchan suji trap is slightly less likely for this reason.
However, in a game with red fives, the player has an incentive to keep the red five. If they have a 135 shape, and the 5 is red, they would likely discard the 1 in order to keep the dora. Or, with a 246 shape, they could take the 46 shape and hope to win on the red five. 456 could be viewed as more dangerous in this ruleset, especially if you can't see the red five in the suit of your 2 or 8. As a side note, this is why the suji of a red five is seen as especially safe.
The dora itself can also change things. If the 1 or 2 is dora, the player would generally want to keep the 13 shape for the value, while if the 4 or 5 is dora, they would want to keep the 35 shape. This applies to the other suji shapes as well.
Checklist
For defense, eighteen different suji are in consideration as listed above. Six basic suji multiplied by the three suits produces the count of eighteen suji. After all, hand shapes frequently depend on ryanmen to win, as they are the most efficient and have the widest range of waiting tiles involving just two tiles in the hand. By counting the number of visible suji, players can determine roughly how risky it is to discard a dangerous tile.
Manzu (1  4) (4  7)  Pinzu (1  4) (4  7)  Souzu (1  4) (4  7) 
Manzu (2  5) (5  8)  Pinzu (2  5) (5  8)  Souzu (2  5) (5  8) 
Manzu (3  6) (6  9)  Pinzu (3  6) (6  9)  Souzu (3  6) (6  9) 
Offense
The strategy of suji may be used offensively. By utilizing suji in reverse, a player may use a tile within an interval to lure out a winning tile along the same interval. A defending player may presume a certain tile to be safe by suji, when it is actually not. This is commonly known as the "suji trap".
A common way for this to occur is when a player has a ryankan shape, such as 468. They can discard the 4 to wait on 7, which is suji of the 4. Other players who know about suji may see this and be more likely to discard the 7. This is part of the reason why the suji of the riichi tile is considered dangerous.
Other suji
Matagi suji
Ura suji
External links
 Suji in Japanese Wikipedia
 Osamuko strategy article on suji
