The article provides guidelines for determining the composition of a structure's existing mortar and advocates using a lime-based mortar for repointing if a lime mortar was used originally. To an architectural preservationist, the worst scenario imaginable is a masonry wall where the mortar joints surround deep impressions, pockets of eroded brick or stone. This indicates that little or no consideration was given to compatibility when the wall was repointed. A dense mortar was used against softer masonry, and the stronger material dominated. Mortar should be flexible enough to absorb movement due to temperature change, settlement or vibration. Lime mortars provide this flexibility. In contrast, when cement is the sole binder used in a mortar, the masonry is forced to absorb any movement and becomes subject to erosion, spalling or cracking. Examining the existing mortar is necessary to achieve the best possible match of the new with the old. Visual examination can yield an abundance of clues. Rub a finger across an existing mortar joint. If the finger picks up white dust, lime is the binder. If the finger appears clean, cement is the binder. Measuring brick sizes is another means of gaining clues to the mortar. Brick was not standardized in size until 1899; before then, brick were made by hand, mold or machine, resulting in many sizes. To supplement visual examination, one can analyze the composition of the existing mortar. These analyses range from simple onsite tests to laboratory studies--all in an effort to determine the type of binder and aggregate originally used. One onsite technique involves placing a pulverized sample in diluted hydrochloric acid (muriatic acid). Vigorous bubbling and an amber hue indicate a lime binder. Weak agitation and a murky green color are characteristic of cements. One fairly simple test to identify binders is visual examination of the dried aggregate. The color of the binder often alters the color characteristics of the undissolved aggregate. For example, the dried residue of portland cement tends to be medium to dark gray, and clay produces a reddish to light tan color. Recently, Scotland has seen a resurgence of lime mortar use and traditional craftsmanship. Our building sector certainly can learn from the example set in Scotland, where the materials required to create lime mortars are readily available. The same cannot be said for North America. Few building supply stores carry lime putty, but it can be obtained from a couple of U.S. sources. And some suppliers will place a special order for hydraulic lime.