Safe Tiles
For example, against the riichi below,
which tiles can you discard with zero chance of being ron'd?
Under the furiten rule, a player cannot ron on any tile they have already discarded.
So every tile in that player's own discard pond is safe.
In this example, that means:
Also, after declaring riichi, if a player passes on a winning tile once, they can no longer ron that tile later.
So in this diagram, the 
discarded by the player across from you and the 
discarded by the player below you are also tiles that cannot deal in.
Tiles that are clearly guaranteed not to deal in because of furiten are called genbutsu.
Besides genbutsu, there is another kind of tile that is 100% safe.
That is the 
you just drew.
Because three copies of this tile are already visible on the table, it can no longer form:
- a shanpon wait
- a tanki wait
And since this cannot be a kokushi musou tenpai, it is a tile that can absolutely never be ron'd: a completely safe tile.
The Four Main Types of Completely Safe Tiles
There are roughly four kinds of completely safe tiles:
- A tile all three other players have each discarded once (in other words, a genbutsu against all three)
- A tile your upper seat has just discarded (this is also a genbutsu against all three)
- The fourth visible honor tile, when kokushi musou is not possible
- A complete no-chance tile
The last category will be explained later on the Kabe page.
In this example, there are plenty of safe tiles.
If you decide to avoid dealing in, doing so is easy.
But in real play, you will not always have this many safe tiles.
That is exactly why you need to learn about suji and kabe next.
Original Japanese page: http://beginners.biz/mamori/mamori02.html