C Generated File by IDA Pro

Question

I have produce a C File from IDA Pro and wanted to know what does the commented register@number notation mean? E.g. ebx@1 right to the int v1; declaration?

Thanks!

int __stdcall sub_421E20(int a1)
{
  int v1; // ebx@1
  int v2; // esi@1
  int v3; // ecx@3
  int v4; // eax@5
  int result; // eax@10
  int v6; // edx@11
  int v7; // [sp+10h] [bp-48h]@5
  int v8; // [sp+28h] [bp-30h]@1
  int *v9; // [sp+2Ch] [bp-2Ch]@5
  char v10; // [sp+30h] [bp-28h]@8
  ....