It's possible for a character to have two skills that work well together. In general, having 5 or more ranks in one skill gives the character a +2 bonus on skill checks with each of its synergistic skills. In some cases, this bonus applies only to specific uses of the skill in question, and not to all checks. Some skills provide benefits on other checks made by a character, such as combat actions.

5 or more ranks in Acrobatics gives a +2 bonus on Athletics checks. 10 or more ranks in Acrobatics gives a +1 bonus to AC, increasing by +1 for every 10 ranks.

5 or more ranks in Athletics gives a +2 bonus on Acrobatics checks.

5 or more ranks in Bluff gives a +2 bonus on Diplomacy checks, Disguise checks, Intimidate checks, and Sleight of Hand checks.

5 or more ranks in Linguistics gives a +2 bonus on Use Magic Device checks.

5 or more ranks in Lore (Arcana) gives a +2 bonus on Spellcraft checks.

5 or more ranks in Lore (Architecture/Engineering) gives a +2 bonus on Search checks.

5 or more ranks in Lore (Dungeoneering) gives a +2 bonus on Survival checks when underground.

5 or more ranks in Lore (Local) gives a +2 bonus on Gather Information checks.

5 or more ranks in Lore (Nature) gives a +2 bonus on Survival checks when aboveground.

5 or more ranks in Lore (Nobility/Royalty) gives a +2 bonus on Diplomacy checks.

5 or more ranks in Search gives a +2 bonus on Survival checks when following tracks.

5 or more ranks in Sense Motive gives a +2 bonus on Diplomacy checks.

5 or more ranks in Spellcraft gives a +2 bonus on Use Magic Device checks and a +1 bonus to spell saving throw for every 5 ranks the character has.

5 or more ranks in Survival gives a +2 bonus on Lore (Nature)_checks.

5 or more ranks in Parry gives a +2 bonus on Dodge AC when using Parry. 25 or more ranks in Parry gives a +6 bonus on Dodge AC when using Parry.

5 or more ranks in Use Magic Device gives a +2 bonus on Spellcraft checks.