Use this if you are using igraph from R

For undirected graphs, head and tail is not defined. In this case
`tail_of`

returns vertices incident to the supplied edges, and
`head_of`

returns the other end(s) of the edge(s).

tail_of(graph, es)

`graph` |
The input graph. |

`es` |
The edges to query. |

A vertex sequence with the tail(s) of the edge(s).

Other structural queries: `[.igraph`

,
`[[.igraph`

, `adjacent_vertices`

,
`are_adjacent`

, `ends`

,
`get.edge.ids`

, `gorder`

,
`gsize`

, `head_of`

,
`incident_edges`

, `incident`

,
`is_directed`

, `neighbors`

[Package *igraph* version 1.2.4.1 Index]