K.13.1- Many cards have an expansion symbol to indicate which expansion set the card is from. This symbol is on the right side of the middle section of the card. [CompRules 2001/07/23 - 206.1]
K.13.2- In newer sets (starting with Exodus in mid 1998), the color of the expansion symbol indicates the rarity of the card. Gold for rare, silver for uncommon, black for common. Older sets (prior to Exodus) always used black for the expansion symbols. [CompRules 2001/07/23 - 206.2]
K.13.3- A spell or ability which affects cards from a certain expansion set only affects ones with the appropriate expansion symbol on it. [CompRules 2001/07/23 - 206.3]
K.13.4- The expansion symbols for various sets are: Limited Edition = No symbol. Unlimited Edition = No symbol. Revised Edition = No symbol. Fourth Edition = No symbol. Fifth Edition = No symbol. Sixth Edition = Roman numeral six ("VI"). Seventh Edition = A stylized "7". Arabian Nights = Scimitar (curved sword). Antiquities = Anvil. Legends = Top of a Greek column. The Dark = A partially eclipsed moon. Fallen Empires = Crown. Ice Age = Snowflake. Homelands = Planet (a circle with blotches on it). Alliances = Flag. Mirage = Palm Tree. Visions = Stylized 'V'. Weatherlight = Open Book. Tempest = Cloud with lightning. Stronghold = Gated Door. Exodus = Bridge. Unglued = Cracked Egg. Urza's Saga = Two Gears. Urza's Legacy = Hammer. Urza's Destiny = Flask. Mercadian Masques = A Mask. Nemesis = An Axe. Prophecy = A set of three crystals. Invasion = A funny shape. Planeshift = A swirl. Apocalypse = A mask with fangs. Odyssey = A crystal ball on a holder (the artifact Mirari). Torment = A snake curled in upon itself. Judgment = A set of scales. Onslaught = A pair of interlocked letters "M". Legions = A shield with two crossed spears behind it. Scourge = A dragon head. Promotional = Dragon. Pen. Stylized 'M'. Feather. DCI symbol. Portal = Circle with radial lines. Portal Second Age = Pentagram with radial lines. Portal Three Kingdoms = Chinese symbol for 3. Deckmasters = A fancy 'D'.