Orthologs & Synteny
Orthologs can sometimes be identified using synteny. For example, below we have 2 related species with 2 chromosomes each.
In deciding which gene a in species 1 might be the ortholog of gene a
in species 2, the additional synteny of genes c and d give supporting
evidence that gene a on chromosome 1 is the orthologous gene. The two
copies of a in species 1 are paralogs, possibly caused by a duplication
event in species 1.