Suji
Read the numbers across. 
Suji 「筋」 is 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.
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.
Of course, one should not be necessarily dependent on these intervals alone, as some waiting patterns are immune to suji.
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".
External links
 Suji in Japanese Wikipedia
 Osamuko strategy article on suji
