One of the customs which we observe during the Nine Days – from Rosh Hodesh Ab through Tisha B'Ab – is to refrain from meat. The Mishna establishes that eating meat is forbidden only during the Se'uda Mafseket – the final meal before Tisha B'Ab – but the custom was accepted to refrain from meat throughout the period of the Nine Days (except, of course, on Shabbat). A number of companies today sell synthetic meat – products made from soy or other ingredients but resemble meat, and are eaten as meat substitutes. Are such products permissible for consumption during the Nine Days? Hacham Bension Abba Shaul (Jerusalem, 1924-1998) writes that although the concept of Mar'it Ha'ayin – avoiding actions which appear as Halachic violations – forbids eating something which appears prohibited, this rule does not apply to something which is forbidden only by force of Minhag (accepted custom). When something is forbidden by the strict Halacha, then we must be concerned about Mar'it Ha'ayin, but not when it is forbidden only on the level of Minhag. Therefore, Hacham Bension writes, it is permissible during the Nine Days to eat vegetarian products which resemble meat, but during the Se'uda Mafseket, when meat is forbidden on the level of strict Halacha, it may be eaten only privately, but not in public view. Summary: Synthetic meat products – meaning, products which resemble meat but are made from soy or other ingredients – may be eaten during the Nine Days. However, during the Se'uda Mafseket right before Tisha B'Ab, such products may be eaten only in private, but not in a public setting.