# Using the BC option

This page describes how to use the `BC`

option to specify boundary conditions at boundary edges of the surface mesh.

Here is an example of a dictionary that can be used with `BC`

:

```
"BC": {1: {"iLow": "ySymm"}, 3: {"jHigh": "splay", "iHigh": "xConst"}}
```

Each entry in the dictionary has a key and at least one nested key-value pair.
The key is the 1-based block number, the nested key is the boundary edge specification, and the value is the boundary condition.
For the first entry, these are `1`

, `iLow`

, and `ySymm`

, respectively.

The 1-based block number and boundary edge specification for a boundary edge can be determined using Tecplot:

Load the surface mesh file into Tecplot.

Use the Probe tool to select a point on the boundary edge of interest. Use Ctrl+click to snap on to a boundary edge.

Select Zone/Cell Info in the toolbar on the right.

The number shown after Zone is the 1-based block number.

The edge specification depends on the values of I and J. The edge is

`iLow`

if I=1,`iHigh`

if I=I-Max,`jLow`

if J=1, or`jHigh`

if J=J-Max. Only one of these is true for any boundary edge.

The supported boundary conditions are:

`splay`

for free edges`xSymm`

,`ySymm`

,`zSymm`

for symmetry planes`xConst`

,`yConst`

,`zConst`

,`xyConst`

,`yzConst`

,`xzConst`

for constant planes