Many types of shields are actual weapons, nothing improvised about it, with their own weapon descriptions:
Light or heavy shield “You can bash an opponent with a shield,”
Klar “A […] klar counts as a light […] shield with armor spikes.” Light shields can bash, so klars can also. “Armor spikes” is presumably a mistake here, since shields get shield spikes (or become “spiked shields”) rather than armor spikes.
Madu “If you are proficient with the madu, you may use it to fight […] If you are not proficient with the madu, treat it as a light spiked shield.” Very weird wording, but whether you’re proficient or not, it counts as something that can be used as a weapon.
Snarlshield “The snarlshield is a heavy shield […] When used as a weapon to bash an opponent,”
Dwarven war-shield “If you have the Two-Weapon Fighting feat and are wielding two dwarven war-shields,” plus the entry has damage and critical hit descriptions, and the Dwarven war-shield appears in the Weapons table.
Other types of shield are not weapons and cannot be used as weapons while worn. You could use them as improvised weapons, but you’d have to unequip them to do that, so you wouldn’t be able to get any of their defensive benefits.
Buckler “You can’t make a shield bash with a buckler.”
Tower shield “You cannot bash with a tower shield,”
Note that heavy shields, light shields, klars, and snarlshields are considered martial weapons, and madus and Dwarven war-shields are exotic weapons. This doesn’t matter for flanking purposes. Flanking requires that you threaten an opponent, and even if you aren’t proficient in the weapon you’re holding, you’re still armed and still able to threaten that opponent. So even a wizard can use a light shield for this purpose.
Also note that a light mithral shield has 0 armor check penalty, and 0% arcane spell failure. Since these are both 0, the shield has no drawbacks even for a nonproficient, arcane-spell-casting wizard, and would still be usable for flanking. (A mithral buckler has the same 0 ACP and 0% ASF, and a hand with a buckler can still hold things so that can be useful, but it cannot bash. Singing steel can also be used to achieve these properties, but is much more expensive.)
:-)– Hey I Can Chan Oct 19 '22 at 15:40